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DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	12 Months Ended
DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	Dec. 31, 2016	Feb. 17, 2017	Jun. 30, 2016
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	0001057352
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 7
Entity Common Stock, Shares Outstanding (in shares)		32,599,696
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenues	$ 837,630	$ 711,764	$ 575,936
Cost of revenues	173,814	188,885	156,979
Gross margin	663,816	522,879	418,957
Operating expenses:
Selling and marketing	296,483	302,226	150,305
Software development	76,400	65,760	55,426
General and administrative	123,297	115,507	103,916
Customer base amortization	22,731	27,931	28,432
Total operating expenses	518,911	511,424	338,079
Income from operations	144,905	11,455	80,878
Interest and other income	1,773	537	516
Interest and other expense	(10,016)	(9,411)	(10,481)
Income before income taxes	136,662	2,581	70,913
Income tax expense, net	51,591	6,046	26,044
Net income (loss)	$ 85,071	$ (3,465)	$ 44,869
Net income (loss) per share — basic (in dollars per share)	$ 2.64	$ (0.11)	$ 1.48
Net income (loss) per share — diluted (in dollars per share)	$ 2.62	$ (0.11)	$ 1.46
Weighted average outstanding shares — basic (in shares)	32,167	31,950	30,215
Weighted average outstanding shares — diluted (in shares)	32,436	31,950	30,641

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 85,071	$ (3,465)	$ 44,869
Other comprehensive loss, net of tax
Foreign currency translation adjustment	(5,032)	(1,466)	(1,690)
Net decrease in unrealized loss on investments	395	256	836
Reclassification adjustment for realized gains on investments included in net income	(808)	0	0
Total other comprehensive loss	(5,445)	(1,210)	(854)
Total comprehensive income (loss)	$ 79,626	$ (4,675)	$ 44,015

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 567,223	$ 421,818
Accounts receivable, net of allowance for doubtful accounts of approximately $7,478 and $6,344 as of December 31, 2015 and 2016, respectively	48,537	40,276
Income tax receivable	129	430
Prepaid expenses and other current assets	11,602	10,209
Total current assets	627,491	472,733
Long-term investments	9,952	15,507
Deferred income taxes, net	7,273	9,107
Property and equipment, net	87,568	88,311
Goodwill	1,254,866	1,252,945
Intangible assets, net	195,965	238,318
Deposits and other assets	1,948	2,650
Total assets	2,185,063	2,079,571
Current liabilities:
Current portion of long-term debt	31,866	16,746
Accounts payable	11,478	9,673
Accrued wages and commissions	33,803	31,045
Accrued expenses	31,092	31,423
Deferred gain on the sale of building	2,523	2,523
Income taxes payable	3,814	46
Deferred rent	1,206	1,687
Deferred revenue	39,164	42,138
Total current liabilities	154,946	135,281
Long-term debt, less current portion	306,473	338,366
Deferred gain on the sale of building	18,715	21,239
Deferred rent	31,589	29,628
Deferred income taxes, net	18,386	4,585
Income taxes payable	741	6,692
Total liabilities	530,850	535,791
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.01 par value; 2,000 shares authorized; none outstanding	0	0
Common stock, $0.01 par value; 60,000 shares authorized; 32,509 and 32,606 issued and outstanding as of December 31, 2015 and 2016, respectively	326	325
Additional paid-in capital	1,471,127	1,440,321
Accumulated other comprehensive loss	(13,039)	(7,594)
Retained earnings	195,799	110,728
Total stockholders’ equity	1,654,213	1,543,780
Total liabilities and stockholders’ equity	$ 2,185,063	$ 2,079,571

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts	$ 6,344	$ 7,478
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
Preferred stock, shares outstanding (in shares)	0	0
Common stock par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued, (in shares)	32,606,000	32,509,000
Common stock, shares outstanding (in shares)	32,606,000	32,509,000

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2013		28,848
Balance at Dec. 31, 2013	$ 927,862	$ 288	$ 863,780	$ (5,530)	$ 69,324
Net income (loss)	44,869				44,869
Other comprehensive loss	(854)			(854)
Exercise of stock options (in shares)		68
Exercise of stock options	3,803	$ 1	3,802
Restricted stock grants (in shares)		260
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(321)
Restricted stock grants surrendered	(50,555)	$ (2)	(50,553)
Stock compensation expense, net of forfeitures	28,503		28,503
Stock issued for equity offering (in shares)		3,450
Stock issued for equity offering	529,360	$ 34	529,326
Employee stock purchase plan (in shares)		13
Employee stock purchase plan	2,152	$ 0	2,152
Excess tax benefit from stock-based compensation	28,406		28,406
Balance (in shares) at Dec. 31, 2014		32,318
Balance at Dec. 31, 2014	1,513,546	$ 323	1,405,414	(6,384)	114,193
Net income (loss)	(3,465)				(3,465)
Other comprehensive loss	(1,210)			(1,210)
Exercise of stock options (in shares)		60
Exercise of stock options	5,069	$ 1	5,068
Restricted stock grants (in shares)		239
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(121)
Restricted stock grants surrendered	(16,436)	$ (1)	(16,435)
Stock compensation expense, net of forfeitures	35,153		35,153
Employee stock purchase plan (in shares)		13
Employee stock purchase plan	2,595	$ 0	2,595
Excess tax benefit from stock-based compensation	8,528		8,528
Balance (in shares) at Dec. 31, 2015		32,509
Balance at Dec. 31, 2015	1,543,780	$ 325	1,440,321	(7,594)	110,728
Net income (loss)	85,071				85,071
Other comprehensive loss	(5,445)			(5,445)
Exercise of stock options (in shares)		29
Exercise of stock options	3,303	$ 0	3,303
Restricted stock grants (in shares)		195
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(142)
Restricted stock grants surrendered	(16,424)	$ (1)	(16,423)
Stock compensation expense, net of forfeitures	36,388		36,388
Employee stock purchase plan (in shares)		15
Employee stock purchase plan	2,842	$ 0	2,842
Excess tax benefit from stock-based compensation	4,698		4,698
Balance (in shares) at Dec. 31, 2016		32,606
Balance at Dec. 31, 2016	$ 1,654,213	$ 326	$ 1,471,127	$ (13,039)	$ 195,799

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities:
Net income (loss)	$ 85,071	$ (3,465)	$ 44,869
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	70,165	78,532	70,372
Amortization of debt issuance costs	3,227	3,311	3,312
Impairment loss	23	2,778	1,799
Loss on disposal of property and equipment	839	681	1,004
Realized gain on investments	(808)	0	0
Excess tax benefit from stock-based compensation	(4,698)	(8,528)	(28,406)
Stock-based compensation expense	36,349	34,537	28,267
Deferred income tax expense (benefit), net	15,635	(5,792)	(1,366)
Provision for losses on accounts receivable	7,358	7,002	4,822
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(16,044)	(3,999)	(12,353)
Income taxes payable	2,816	11,380	24,542
Prepaid expenses and other current assets	(1,157)	367	(2,846)
Deposits and other assets	758	686	(157)
Accounts payable and other liabilities	(1,520)	9,938	6,078
Deferred revenue	(2,070)	3,817	3,972
Net cash provided by operating activities	195,944	131,245	143,909
Investing activities:
Proceeds from sale and settlement of investments	5,950	1,900	5,675
Purchases of property and equipment and other assets	(18,766)	(35,061)	(27,444)
Acquisitions, net of cash acquired	(10,443)	(182,341)	(584,218)
Net cash used in investing activities	(23,259)	(215,502)	(605,987)
Financing activities:
Proceeds from long-term debt	0	0	550,000
Payments of long-term debt	(20,000)	(20,000)	(318,125)
Payments of debt issuance costs	0	0	(9,969)
Payments of deferred consideration	0	0	(1,344)
Excess tax benefit from stock-based compensation	4,698	8,528	28,406
Repurchase of restricted stock to satisfy tax withholding obligations	(16,424)	(16,436)	(50,555)
Proceeds from equity offering, net of transaction costs	0	0	529,360
Proceeds from exercise of stock options and employee stock purchase plan	5,861	7,404	5,740
Net cash provided by (used in) financing activities	(25,865)	(20,504)	733,513
Effect of foreign currency exchange rates on cash and cash equivalents	(1,415)	(433)	(376)
Net increase (decrease) in cash and cash equivalents	145,405	(105,194)	271,059
Cash and cash equivalents at beginning of year	421,818	527,012	255,953
Cash and cash equivalents at end of year	$ 567,223	$ 421,818	$ 527,012

ORGANIZATION	12 Months Ended
ORGANIZATION	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION	ORGANIZATION CoStar Group, Inc. (the “Company” or “CoStar”) provides information, analytics and online marketplace services to the commercial real estate and related business community through its comprehensive, proprietary database of commercial real estate information covering the United States (“U.S.”), the United Kingdom (“U.K.”), and parts of Canada, Spain, Germany and France. The Company provides online marketplaces for commercial real estate, apartment rentals, lands for sale and businesses for sale. The Company operates within two operating segments, North America and International, and its services are typically distributed to its clients under subscription-based license agreements that renew automatically, a majority of which have a term of one year.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment.

Use of Estimates

The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities (“ARS”), accounting for business combinations and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenues and expenses. Actual results could differ from these estimates.

Revenue Recognition

The Company primarily derives revenues by providing access to its proprietary database of commercial real estate information. The Company generally charges a fixed monthly amount for its subscription-based services. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically.

Revenues are recognized when (1) there is persuasive evidence of an arrangement, (2) the fee is fixed and determinable, (3) services have been rendered and payment has been contractually earned and (4) collectability is reasonably assured.

Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. Deferred revenue results from advance cash receipts from customers or amounts billed in advance to customers from the sale of subscription licenses and is recognized over the term of the license agreement.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Revenue Recognition — (Continued)

The Company analyzes contracts with multiple elements under the accounting guidance for multiple-element arrangements. The Company's multiple-element arrangements include information, analytics and/or online marketplace services that are generally provided to the customer over the same term. When identifying multiple-element arrangements, the Company considers multiple purchases made by the same customer within a short time frame and assesses whether the purchases were negotiated together as one overall arrangement. If a multiple-element arrangement is identified, then the arrangement consideration is allocated among the separate units of accounting based on their relative selling prices, which are estimated considering factors such as historical pricing, pricing strategy, market conditions and other factors. The Company accounts for each deliverable in the transaction separately. If the deliverables cannot be separated into multiple units of accounting, then the arrangement consideration is combined and recognition of revenue is determined for the combined unit of accounting. Multiple-element transactions require judgment to determine the selling price or fair value of the different elements. These judgments impact the amount of revenue recognized over the term of the contract, as well as the period in which they are recognized.

Cost of Revenues

Cost of revenues principally consists of salaries, benefits, bonuses, and stock-based compensation expenses for the Company’s researchers who collect and analyze the commercial real estate data that is the basis for the Company’s information, analytics and online marketplaces. Additionally, cost of revenues includes the cost of data from third-party data sources, credit card and other transaction fees relating to processing customer transactions, which are expensed as incurred, and the amortization of acquired trade names and other intangible assets and database technology.

Foreign Currency Translation

The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Currency gains and losses on the translation of intercompany loans made to foreign subsidiaries that are of a long-term investment nature are also included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations. There were no material gains or losses from foreign currency exchange transactions for the years ended December 31, 2014, 2015 and 2016.

Accumulated Other Comprehensive Loss

The components of accumulated other comprehensive loss were as follows (in thousands):



	As of December 31,
	2015			2016
Foreign currency translation adjustment	$	(7,159	)	$	(12,191	)
Net unrealized loss on investments, net of tax	(435		)	(848		)
Total accumulated other comprehensive loss	$	(7,594	)	$	(13,039	)

The amount of realized gain from the redemption of available-for-sale securities reclassified out of accumulated other comprehensive loss to the consolidated statement of operations for the year ended December 31, 2016 was approximately $808,000. There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statements of operations for the years ended December 31, 2014 and 2015, respectively.

Advertising Costs

The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising. Advertising costs were approximately $29 million, $132 million and $109 million for the years ended December 31, 2014, 2015 and 2016, respectively.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Income Taxes

Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if the Company anticipates that some or all of an asset may not be realized through future taxable earnings or implementation of tax planning strategies. Interest and penalties related to income tax matters are recognized in income tax expense. See Note 10 for additional information regarding income taxes.

Net Income (Loss) Per Share

Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect.

Stock-Based Compensation

Equity instruments issued in exchange for services performed by officers, employees, directors and consultants of the Company are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations.

Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed.

In February 2014, March 2015 and March 2016, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by the end of each respective three-year period. These grants of restricted common stock are also subject to continuing employment requirements and a market condition based on total shareholder return (“TSR”). The actual number of shares that vest at the end of each respective three-year period is determined based on the Company’s achievement of the three-year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. As of December 31, 2016, the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2014, 2015 and 2016 awards would be met by their respective forfeiture dates. As a result, the Company recorded a total of approximately $1 million, $3 million and $3 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2014, 2015 and 2016, respectively. The Company expects to record estimated stock-based compensation expense related to the performance-based restricted common stock awards of approximately $5 million over the periods 2017, 2018 and 2019.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Stock-Based Compensation — (Continued)

Cash flows resulting from excess tax benefits are classified as part of cash flows from operating and financing activities. Excess tax benefits represent tax benefits for stock-based compensation in excess of the associated deferred tax asset for such equity compensation recorded as an increase to stockholders' equity. Net cash proceeds from the exercise of stock options and the purchase of shares under the Employee Stock Purchase Plan (“ESPP”) were approximately $6 million, $7 million and $6 million for the years ended December 31, 2014, 2015 and 2016, respectively. The Company realized approximately $28 million, $9 million and $5 million of excess tax benefits from stock options exercised and restricted stock awards vested for the years ended December 31, 2014, 2015 and 2016, respectively.

Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the ESPP included in the Company’s results of operations were as follows (in thousands):



	Year Ended December 31,
	2014		2015		2016
Cost of revenues	$	4,759	$	5,815	$	5,495
Selling and marketing	3,776		5,114		6,634
Software development	5,095		5,712		6,546
General and administrative	14,637		17,896		17,674
Total stock-based compensation	$	28,267	$	34,537	$	36,349

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. Cash equivalents consist of money market fund investments and commercial paper.

Investments

The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities. Investments are carried at fair value. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned.

Concentration of Credit Risk and Financial Instruments

The Company performs ongoing assessments of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for estimated inherent credit losses, and such losses have been within management’s expectations. The large size and widespread nature of the Company’s customer base and the Company’s lack of dependence on any individual customer mitigates the risk of nonpayment of the Company’s accounts receivable. No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2014, 2015 and 2016. The carrying amount of the accounts receivable approximates the net realizable value. The carrying value of the accounts receivable, accounts payable, accrued expenses and long-term debt approximates fair value.

The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. The carrying value of cash approximates fair value. Historically, the Company has not experienced any losses due to such cash concentrations.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Accounts Receivable, Net of Allowance for Doubtful Accounts

Accounts receivable are recorded at the invoiced amount. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. When evaluating the adequacy of this allowance for doubtful accounts, the Company analyzes historical collection experience, changes in customer payment profiles and the aging of receivable balances, as well as current economic conditions, all of which may affect a customer’s ability to pay.

Property and Equipment, Net

Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets:



Leasehold improvements		Shorter of lease term or useful life
Furniture and office equipment		Five to ten years
Vehicles		Five to ten years
Computer hardware and software		Two to five years

Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred.

Goodwill and Intangible Assets

Goodwill represents the excess of costs over the fair value of assets of acquired businesses. Goodwill is not amortized, but instead tested for impairment at least annually by each reporting unit. To determine whether it is necessary to perform the two-step goodwill impairment test, the Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If the Company concludes that it is more likely than not that the fair value of a reporting unit is less than its carrying amount or if the Company elects not to assess qualitative factors, then the Company performs the two-step process. The first step is to determine the fair value of each reporting unit. The estimate of the fair value of each reporting unit is based on a projected discounted cash flow model that includes significant assumptions and estimates including the Company's discount rate, growth rate and future financial performance. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the growth rate and future financial performance of a reporting unit are based on the Company's forecasts, business plans, economic projections and anticipated future cash flows. The fair value of each reporting unit is compared to the carrying amount of the reporting unit. If the carrying value of the reporting unit exceeds the fair value, then the second step of the process is performed to measure the impairment loss.

Intangible assets with estimable useful lives that arose from acquisitions on or after July 1, 2001 are amortized over their respective estimated useful lives using a method of amortization that reflects the pattern in which the economic benefits of the intangible assets are consumed or otherwise used up. Intangible assets are reviewed for impairment at least annually, and more frequently whenever events or changes in circumstances indicate that the carrying value may not be recoverable.

Acquired database technology, customer base and trade names and other intangible assets are related to the Company’s acquisitions (see Notes 3, 7 and 8). Acquired database technology is amortized on a straight-line basis over periods ranging from three years to eight years. Acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from three years to fifteen years. See Note 8 for further details on the reclassification of the acquired trade names recorded in connection with the LoopNet acquisition from an indefinite-lived intangible asset to a definite-lived intangible asset. Acquired intangible assets characterized as customer base consists of acquired customer contracts and the related customer relationships and are amortized over periods ranging from ten years to thirteen years. Acquired customer bases are typically amortized on an accelerated basis related to the expected economic benefit of the intangible asset. The cost of capitalized building photography is amortized on a straight-line basis over periods ranging from three years to five years.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Long-Lived Assets

Long-lived assets, such as property and equipment, and purchased intangibles subject to amortization, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset.

Assets to be disposed of would be separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and would no longer be depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the balance sheet.

Debt Issuance Costs

Costs incurred in connection with the issuance of long-term debt are capitalized and amortized as interest expense over the term of the related debt using the effective interest method. These amounts are reflected in the consolidated balance sheets as direct deductions from a combination of the current and long-term portions of debt. Upon a refinancing, previously capitalized debt issuance costs are expensed and included in loss on extinguishment of debt if the Company determines that there has been a substantial modification of the related debt. If the Company determines that there has not been a substantial modification of the related debt, any previously capitalized debt issuance costs are amortized as interest expense over the term of the new debt instrument using the effective interest method. The Company had capitalized debt issuance costs, net of amortization, of approximately $10 million and $7 million as of December 31, 2015 and 2016, respectively. The debt issuance costs are associated with the financing commitment received from JPMorgan Chase Bank, N.A. (“J.P. Morgan Bank”) on April 27, 2011, the subsequent term loan facility and revolving credit facility established under a credit agreement dated February 16, 2012 (the “2012 Credit Agreement”), the financing commitment received from J.P. Morgan Bank, Bank of America, N.A., SunTrust Bank and Wells Fargo Bank, National Association on February 28, 2014, and the subsequent term loan facility and revolving credit facility established under a credit agreement dated April 1, 2014 (the “2014 Credit Agreement”). See Note 9 for additional information regarding the term loan facility and revolving credit facility. The Company amortized debt issuance costs of approximately $3 million for each of the years ended December 31, 2014, 2015 and 2016.

Business Combinations

The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. See Note 3 for additional information regarding the Company's recent business combinations.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard that is designed to improve financial reporting by creating common recognition guidance for GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides useful information to users of financial statements through improved disclosure requirements and simplifies the presentation of financial statements. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. From March to December 2016, amendments to the new revenue recognition standard were issued to clarify numerous accounting topics, including, but not limited to (i) the implementation guidance on principal versus agent considerations, (ii) the identification of performance obligations, (iii) the licensing implementation guidance, (iv) the objective of the collectability criterion, (v) the application of the variable consideration guidance and modified retrospective transition method, (vi) the way in which impairment testing is performed and (vii) the disclosure requirements for revenue recognized from performance obligations. This guidance permits the use of either a full retrospective method or a modified retrospective approach. The modified retrospective approach would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. This guidance will be effective for annual reporting periods beginning after December 15, 2017, although companies may adopt the standard as early as annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. The Company is currently evaluating the potential impact of adopting this guidance and, although it expects significant changes to its financial statement disclosures, due to the nature of its services and current revenue recognition practices, it does not believe the new revenue recognition standard will have a material impact on its revenue recognition policies. The Company expects adoption of the new standard will change its current treatment of commissions paid to employees which the Company currently expenses as incurred. Under the new standard, the Company expects to capitalize commission costs as an incremental cost of obtaining a contract, which may have a material impact on the Company's financial statements. The Company is currently evaluating the period over which the capitalized commission costs will be amortized. The Company is still assessing the financial statement impact of capitalizing commission costs and as a result, it has not yet selected a transition method, but it expects to adopt the new standard on January 1, 2018.

In February 2016, the FASB issued authoritative guidance to increase transparency and comparability among organizations’ accounting for leases. The guidance requires a company to recognize lease assets and lease liabilities on the balance sheet, as well as disclose key information about leasing arrangements. This guidance is effective on a modified retrospective basis for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period. Early application is permitted. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures, but expects that the adoption of this standard may result in a material increase in assets and liabilities on its consolidated balance sheets.

In March 2016, the FASB issued authoritative guidance to simplify several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification of share-based payment transactions on the statement of cash flows. The guidance requires a company to (i) recognize all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations using a prospective transition method, (ii) recognize excess tax benefits in the current period regardless of whether the benefit reduces taxes payable using a modified retrospective transition method, and (iii) classify all excess tax benefits as operating activities within the statement of cash flows using either a prospective transition method or a retrospective transition method. The guidance also allows a company to (i) elect whether to estimate the number of share-based awards expected to vest or account for forfeitures when they occur, and (ii) withhold up to the maximum statutory tax rate in the applicable jurisdiction for awards, both of which should be applied using a modified retrospective transition method. Finally, the guidance requires a company to classify the cash paid by an employer when directly withholding shares for tax withholding purposes as a financing activity within the statement of cash flows using a retrospective transition method. This guidance is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early application is permitted. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures, but expects that the adoption of this standard will impact the Company’s net income because excess tax benefits, which are currently reflected in additional paid in capital, will be reflected in income tax expense. The significance of the impact will depend on the intrinsic value at the time of vesting or exercise of equity instruments. The adoption of this standard will also impact the consolidated statements of cash flows, as excess tax benefits will be presented as an operating activity instead of a financing activity. Finally, the Company plans to continue to estimate the number of awards expected to be forfeited and adjust the estimate when it is no longer probable that the service or performance conditions will be met.


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Recent Accounting Pronouncements — (Continued)

In June 2016, the FASB issued authoritative guidance designed to provide financial statement users with more information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. When determining such expected credit losses, the guidance requires companies to apply a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. This guidance is effective on a modified retrospective basis for annual reporting periods beginning after December 15, 2019, including interim periods within that reporting period. Companies may adopt the standard as early as annual reporting periods beginning after December 15, 2018. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures.

In August 2016, the FASB issued authoritative guidance designed to reduce the existing diversity in how certain cash receipts and cash payments are presented and classified in the consolidated statements of cash flows. This guidance is effective on a retrospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. Early application is permitted. This guidance is not expected to have a material impact on the Company's consolidated statements of cash flows and related disclosures.

In January 2017, the FASB issued authoritative guidance designed to clarify the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance indicates that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets is not a business. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. Early application is permitted in certain situations. This guidance is not expected to have a material impact on the Company's consolidated financial statements and related disclosures.

In January 2017, the FASB issued authoritative guidance designed to simplify the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. The guidance indicates that an entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2019, including interim periods within that reporting period. Early application is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. This guidance is not expected to have a material impact on the Company's consolidated financial statements and related disclosures.

ACQUISITION

12 Months Ended

Dec. 31, 2016

Business Combinations [Abstract]

ACQUISITIONS

ACQUISITION

Pursuant to the definitive agreement and plan of merger with Network Communications, Inc. (“NCI”) dated April 27, 2015 (the “Merger Agreement”), on June 1, 2015, the Company acquired 100% of the outstanding stock of NCI and the related Apartment Finder business (collectively referred to as “Apartment Finder”) from the former stockholders of NCI. Apartment Finder provides lead generation, advertising and Internet marketing solutions to property managers and owners through its main service, ApartmentFinder.com^TM. The acquisition furthered the Company's expansion into the multifamily vertical. The Apartment Finder acquisition was not deemed to be a significant acquisition.

In consideration for the purchase of Apartment Finder, on June 1, 2015, the Company paid approximately $173 million in cash, including an estimated $3 million in connection with a preliminary adjustment for net working capital as of the closing date. Pursuant to the terms of the Merger Agreement, the purchase price was increased by approximately $21,000 following the final determination of the net working capital of NCI as of the closing date, and this amount was paid to NCI in the third quarter of 2015.


3.	ACQUISITION — (CONTINUED)

The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date. The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands):



Cash and cash equivalents	$	39
Accounts receivable	4,556
Goodwill	107,692
Acquired trade names and other intangible assets	23,642
Acquired customer base	21,856
Acquired database technology	4,076
Acquired building photography	2,425
Deferred income taxes, net	9,290
Other assets and liabilities	(849		)
Fair value of identifiable net assets acquired	$	172,727

The net assets of Apartment Finder were recorded at their estimated fair value. In valuing acquired assets and liabilities, fair value estimates were based on, but were not limited to, future expected cash flows, market rate assumptions for contractual obligations, and appropriate discount rates.

The acquired customer base for the acquisition consisted of three distinct intangible assets, is composed of acquired customer contracts and the related customer relationships, and has a weighted average estimated useful life of ten years. The acquired database technology had an estimated useful life of five months due to the Company's intent to replace the acquired database technology in 2015, which it did in December 2015. The acquired trade names and other intangible assets have a weighted average estimated useful life of nine years. The acquired building photography had an estimated useful life of five months. Amortization of the acquired customer base is recognized on an accelerated basis related to the expected economic benefit of the intangible asset, while amortization of the acquired database technology, acquired building photography and acquired trade names and other intangible assets is recognized on a straight-line basis over their respective estimated useful lives. Goodwill recorded in connection with this acquisition is not amortized, but is subject to annual impairment tests. The approximately $108 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment. None of the goodwill recognized is expected to be deductible for income tax purposes in future periods.

Goodwill is calculated as the excess of the consideration transferred over the net assets recognized and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Specifically, the goodwill recorded as part of the Apartment Finder acquisition includes, but is not limited to: (i) the expected synergies and other benefits that the Company believes will result from combining its operations with Apartment Finder's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce.

- Definition

The entire disclosure for a business combination (or series of individually immaterial business combinations) completed during the period, including background, timing, and recognized assets and liabilities. The disclosure may include leverage buyout transactions (as applicable).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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+ Details

INVESTMENTS

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

INVESTMENTS


4.	INVESTMENTS — (CONTINUED)

Scheduled maturities of investments classified as available-for-sale as of December 31, 2016 are as follows (in thousands):



Maturity	Fair Value
Due in:
2017	$	—
2018 — 2021	—
2022 — 2026	—
2027 and thereafter	9,952
Available-for-sale investments	$	9,952

The Company realized gains of $808,000 on its investments during the year ended December 31, 2016, related to an ARS acquired in a prior acquisition that was redeemed at a par value of $1 million in the fourth quarter of 2016. The Company had no realized gains on its investments for the years ended December 31, 2014 and 2015. The Company had no realized losses on its investments for the years ended December 31, 2014, 2015 and 2016. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis.

As of December 31, 2016, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Auction rate securities	$	10,800	$	—	$	(848	)	$	9,952
Available-for-sale investments	$	10,800	$	—	$	(848	)	$	9,952

As of December 31, 2015, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Auction rate securities	$	15,942	$	610	$	(1,045	)	$	15,507
Available-for-sale investments	$	15,942	$	610	$	(1,045	)	$	15,507

The unrealized losses on the Company’s investments as of December 31, 2015 and 2016 were generated primarily from changes in interest rates and ARS that failed to settle at auction, due to adverse conditions in the global credit markets. The losses are considered temporary, as the contractual terms of these investments do not permit the issuer to settle the security at a price less than the amortized cost of the investment. Because the Company does not intend to sell these instruments and it is not more likely than not that the Company will be required to sell these instruments prior to anticipated recovery, which may be at maturity, the Company does not consider these investments to be other-than-temporarily impaired as of December 31, 2015 and 2016. See Note 5 for further discussion of the fair value of the Company’s financial assets.


4.	INVESTMENTS — (CONTINUED)

The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands):



	December 31,
	2015					2016
	Aggregate Fair Value		Gross Unrealized Losses			Aggregate Fair Value		Gross Unrealized Losses
Auction rate securities	$	14,455	$	(1,045	)	$	9,952	$	(848	)
Investments in an unrealized loss position	$	14,455	$	(1,045	)	$	9,952	$	(848	)

The Company did not have any investments in an unrealized loss position for less than twelve months as of December 31, 2015 and 2016, respectively.

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

FAIR VALUE

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

FAIR VALUE

Fair value is defined as the price that would be received in the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. There is a three-tier fair value hierarchy, which categorizes the inputs used in measuring fair value. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets for identical assets or liabilities; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable; and Level 3, defined as unobservable inputs for which little or no market data exists, therefore requiring an entity to develop its own assumptions.

The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2016 (in thousands):



	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	175,344	$	—	$	—	$	175,344
Commercial paper	6,383		—		—		6,383
Auction rate securities	—		—		9,952		9,952
Total assets measured at fair value	$	181,727	$	—	$	9,952	$	191,679



	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,043	$	—	$	—	$	5,043
Commercial paper	11,178		—		—		11,178
Auction rate securities	—		—		15,507		15,507
Total assets measured at fair value	$	16,221	$	—	$	15,507	$	31,728

The Company’s Level 3 assets consist of ARS, whose underlying assets are primarily student loan securities supported by guarantees from the Federal Family Education Loan Program (“FFELP”) of the U.S. Department of Education.


5.	FAIR VALUE — (CONTINUED)

The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2014 to December 31, 2016 (in thousands):



	Auction Rate Securities
Balance at December 31, 2014	$	17,151
Decrease in unrealized loss included in accumulated other comprehensive loss	256
Settlements	(1,900		)
Balance at December 31, 2015	15,507
Decrease in unrealized loss included in accumulated other comprehensive loss	395
Settlements	(5,950		)
Balance at December 31, 2016	$	9,952

ARS are variable rate debt instruments whose interest rates are reset approximately every 28 days. The underlying securities have contractual maturities greater than twenty years. The ARS are recorded at fair value.

As of December 31, 2016, the Company held ARS with $11 million par value, all of which failed to settle at auction. The majority of these investments are of high credit quality with AAA credit ratings and are primarily student loan securities supported by guarantees from the FFELP of the U.S. Department of Education. The Company may not be able to liquidate and fully recover the carrying value of the ARS in the near term. As a result, these securities are classified as long-term investments in the Company’s consolidated balance sheet as of December 31, 2016. See Note 4 for further discussion of the scheduled maturities of investments classified as available-for-sale.

While the Company continues to earn interest on its ARS investments at the contractual rate, these investments are not currently actively trading and therefore do not currently have a readily determinable market value. The estimated fair value of the ARS no longer approximates par value. The Company used a discounted cash flow model to determine the estimated fair value of its investment in ARS as of December 31, 2016. The assumptions used in preparing the discounted cash flow model include estimates for interest rates, credit spreads, timing and amount of contractual cash flows, liquidity risk premiums, expected holding periods and default risk. The Company updates the discounted cash flow model on a quarterly basis to reflect any changes in the assumptions used in the model and settlements of ARS investments that occurred during the period.

The only significant unobservable input in the discounted cash flow model is the discount rate. The discount rate used represents the Company's estimate of the yield expected by a market participant from the ARS investments. The weighted average discount rate used in the discounted cash flow model as of December 31, 2015 and 2016 was approximately 5%. Selecting another discount rate within the range used in the discounted cash flow model would not result in a significant change to the fair value of the ARS.

Based on this assessment of fair value, as of December 31, 2016, the Company determined there was a decline in the fair value of its ARS investments of approximately $848,000. The decline was deemed to be a temporary impairment and recorded as an unrealized loss in accumulated other comprehensive loss in stockholders’ equity. In addition, while a majority of the ARS are currently rated AAA, if the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, the Company may be required to record additional unrealized losses in accumulated other comprehensive loss or an other-than-temporary impairment charge to earnings on these investments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT

Property and equipment consists of the following (in thousands):



	December 31,
	2015			2016
Leasehold improvements	$	49,752		$	53,073
Furniture, office equipment and vehicles	38,129			45,035
Computer hardware and software	56,656			64,577
Property and equipment, gross	144,537			162,685
Accumulated depreciation and amortization	(56,226		)	(75,117		)
Property and equipment, net	$	88,311		$	87,568

Depreciation expense for property and equipment was approximately $15 million, $20 million and $24 million for the years ended December 31, 2014, 2015 and 2016, respectively.

GOODWILL

12 Months Ended

Dec. 31, 2016

Goodwill [Abstract]

GOODWILL

The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands):



	North America		International			Total
Goodwill, December 31, 2014	$	1,114,363	$	24,442		$	1,138,805
Acquisition	112,947		2,400			115,347
Effect of foreign currency translation	—		(1,207		)	(1,207		)
Goodwill, December 31, 2015	1,227,310		25,635			1,252,945
Acquisitions	467		5,933			6,400
Effect of foreign currency translation	—		(4,479		)	(4,479		)
Goodwill, December 31, 2016	$	1,227,777	$	27,089		$	1,254,866

The Company recorded goodwill of approximately $108 million in connection with the June 1, 2015 acquisition of Apartment Finder and recorded goodwill of approximately $2 million in connection with the July 1, 2015 acquisition of the assets of Belbex Corporate, S.L., a commercial real estate information provider operating in Madrid, Spain. Additionally, the Company recorded goodwill of approximately $5 million and $467,000 during the years ended December 31, 2015 and 2016, respectively, in connection with the acquisition of certain assets related to the business operations of Apartment Finder's independent distributors within various markets. Finally, the Company recorded goodwill of approximately $6 million in connection with the May 3, 2016 acquisition of Thomas Daily GmbH (“Thomas Daily”), a commercial real estate news and information provider operating in Freiburg, Germany.

During the fourth quarters of 2014, 2015 and 2016, the Company completed the annual impairment test of goodwill and concluded that goodwill was not impaired.

INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2016

Intangible Assets, Net (Excluding Goodwill) [Abstract]

INTANGIBLE ASSETS

Intangible assets consist of the following (in thousands, except amortization period data):



	December 31,						Weighted- Average Amortization Period (in years)
	2015			2016			Weighted- Average Amortization Period (in years)
Capitalized product development cost	$	2,243		$	2,275		4
Accumulated amortization	(2,172		)	(2,217		)
Capitalized product development cost, net	71			58

Building photography	17,677			17,271			4
Accumulated amortization	(15,875		)	(16,351		)
Building photography, net	1,802			920

Acquired database technology	77,905			78,151			5
Accumulated amortization	(62,818		)	(72,691		)
Acquired database technology, net	15,087			5,460

Acquired customer base	221,409			220,749			10
Accumulated amortization	(129,782		)	(150,445		)
Acquired customer base, net	91,627			70,304

Acquired trade names and other intangible assets	153,910			153,607			13
Accumulated amortization	(24,179		)	(34,384		)
Acquired trade names and other intangible assets, net	129,731			119,223

Intangible assets, net	$	238,318		$	195,965

Amortization expense for intangible assets was approximately $55 million, $59 million and $46 million for the years ended December 31, 2014, 2015 and 2016, respectively.

In the aggregate, the Company expects amortization for intangible assets existing as of December 31, 2016 for future periods to be approximately $33 million, $24 million, $21 million, $19 million and $18 million for the years ending December 31, 2017, 2018, 2019, 2020 and 2021, respectively.

Intangible assets are reviewed for impairment at least annually and more frequently whenever events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. During the first quarter of 2016, the Company determined that the acquired trade names recorded in connection with the LoopNet acquisition on April 30, 2012 should be reclassified from an indefinite-lived intangible asset to a definite-lived intangible asset due to work being performed to integrate the backend systems of LoopNet and CoStar, which may result in a future re-branding effort if aspects of the LoopNet and CoStar services are ultimately combined. The Company estimated the fair value of the LoopNet trade names using the relief from royalty method and concluded that no impairment existed as of March 31, 2016. The Company estimated a useful life of fifteen years for the LoopNet trade names, which are being amortized on a straight-line basis.

LONG-TERM DEBT

12 Months Ended

Dec. 31, 2016

Long-term Debt, Current and Noncurrent [Abstract]

LONG-TERM DEBT

On April 1, 2014 (the “Closing Date”), the Company entered into the 2014 Credit Agreement by and among the Company, as Borrower, CoStar Realty Information, Inc., as Co-Borrower, the Lenders from time to time party thereto and JPMorgan Chase Bank, N.A., as Administrative Agent. The 2014 Credit Agreement provides for a $400 million term loan facility and a $225 million revolving credit facility, each with a term of five years. The proceeds of the term loan facility and the initial borrowing of $150 million under the revolving credit facility on the Closing Date were used to refinance the 2012 Credit Agreement, including related fees and expenses, and to pay a portion of the consideration and transaction costs related to the acquisition of Apartments.com. The undrawn proceeds of the revolving credit facility are available for the Company's working capital needs and other general corporate purposes. During June 2014, the Company repaid the $150 million initial borrowing under the revolving credit facility. The carrying value of the term loan facility approximates fair value and can be estimated through Level 3 unobservable inputs using a valuation technique based on expected cash flows discounted using the current credit-adjusted risk-free rate, which approximates the rate of interest on the term loan facility at origination.

Effective April 1, 2014, the Company terminated the 2012 Credit Agreement and repaid all amounts outstanding thereunder, which amounts totaled approximately $149 million. The Company evaluated the execution of the 2014 Credit Agreement and the termination of the 2012 Credit Agreement and determined that the transactions did not qualify as an extinguishment of debt because the change in the present value of future cash flows between the initial term loan facility under the 2012 Credit Agreement and the new term loan facility under the 2014 Credit Agreement was not considered a substantial modification.

The revolving credit facility includes a subfacility for swingline loans of up to $10 million, and up to $10 million of the revolving credit facility is available for the issuance of letters of credit. The term loan facility will amortize in quarterly installments in amounts resulting in an annual amortization of 5% during each of the first, second and third years, 10% during the fourth year and 15% during the fifth year after the Closing Date, with the remainder payable at final maturity. The loans under the 2014 Credit Agreement bear interest, at the Company's option, either (i) during any interest period selected by the Company, at the London interbank offered rate for deposits in U.S. dollars with a maturity comparable to such interest period, adjusted for statutory reserves (“LIBOR”), plus an initial spread of 2% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2014 Credit Agreement) of the Company, or (ii) at the greatest of (x) the prime rate from time to time announced by JPMorgan Chase Bank, N.A., (y) the federal funds effective rate plus 0.5% and (z) LIBOR for a one-month interest period plus 1%, plus an initial spread of 1% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurs under the 2014 Credit Agreement, the interest rate on overdue amounts will increase by 2% per annum. The obligations under the 2014 Credit Agreement are guaranteed by all material subsidiaries of the Company and are secured by a lien on substantially all of the assets of the Company and those of its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee documents entered into on the Closing Date.

The 2014 Credit Agreement requires the Company to maintain (i) a First Lien Secured Leverage Ratio (as defined in the 2014 Credit Agreement) not exceeding 3.5 to 1.0 during the three months ended December 31, 2016, and each full fiscal quarter thereafter and (ii) after the incurrence of additional indebtedness under certain specified exceptions in the 2014 Credit Agreement, a Total Leverage Ratio (as defined in the 2014 Credit Agreement) not exceeding 4.5 to 1.0 during the three months ended December 31, 2016, and each full fiscal quarter thereafter. The 2014 Credit Agreement also includes other covenants, including covenants that, subject to certain exceptions, restrict the ability of the Company and its subsidiaries to (i) incur additional indebtedness, (ii) create, incur, assume or permit to exist any liens, (iii) enter into mergers, consolidations or similar transactions, (iv) make investments and acquisitions, (v) make certain dispositions of assets, (vi) make dividends, distributions and prepayments of certain indebtedness, and (vii) enter into certain transactions with affiliates. The Company was in compliance with the covenants in the 2014 Credit Agreement as of December 31, 2016.

In connection with obtaining the term loan facility and revolving credit facility pursuant to the 2014 Credit Agreement, the Company incurred approximately $10 million in debt issuance costs as of April 1, 2014, which along with the unamortized debt issuance cost from the 2012 Credit Agreement, were capitalized and are amortized as interest expense over the term of the 2014 Credit Agreement using the effective interest method.

As of December 31, 2015 and 2016, no amounts were outstanding under the revolving credit facility. Total interest expense for the term loan facility and revolving credit facility was approximately $10 million, $9 million and $10 million for the years ended December 31, 2014, 2015 and 2016, respectively. Interest expense included amortized debt issuance costs of approximately $3 million for each of the years ended December 31, 2014, 2015 and 2016. Total interest paid for the term loan facility and revolving credit facility was approximately $7 million, $6 million and $7 million for the years ended December 31, 2014, 2015 and 2016, respectively.


9.	LONG-TERM DEBT — (CONTINUED)

The following table represents the Company's long-term debt (in thousands):



	December 31,
	2015			2016
Term loan facility	$	365,000		$	345,000
Debt issuance costs, net	(9,888		)	(6,661		)
Total debt	355,112			338,339
Current maturities of long-term debt	(20,000		)	(35,000		)
Current debt issuance costs, net	3,254			3,134
Total long-term debt, less current portion	$	338,366		$	306,473

Maturities of the Company's borrowings under the 2014 Credit Agreement for each of the next three years as of December 31, 2016 are as follows (in thousands):



Year ending December 31,	Maturities
Due in:
2017	$	35,000
2018	55,000
2019	255,000
Long-term debt, including current maturities	$	345,000

INCOME TAXES

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

INCOME TAXES

The components of the provision for income taxes attributable to operations consist of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Current:
Federal	$	24,741		$	10,295		$	32,198
State	2,761			1,503			3,682
Foreign	53			40			76
Total current	27,555			11,838			35,956
Deferred:
Federal	(698		)	(7,475		)	13,641
State	(813		)	1,683			1,959
Foreign	—			—			35
Total deferred	(1,511		)	(5,792		)	15,635
Total provision for income taxes	$	26,044		$	6,046		$	51,591


10.	INCOME TAXES — (CONTINUED)

The components of deferred tax assets and liabilities consist of the following (in thousands):



	December 31,
	2015			2016
Deferred tax assets:
Reserve for bad debts	$	2,849		$	2,437
Accrued compensation	10,725			5,562
Stock compensation	12,686			14,268
Net operating losses	36,406			30,319
Accrued reserve and other	3,515			2,097
Unrealized loss on securities	377			326
Deferred rent	7,274			7,814
Deferred revenue	2,243			—
Deferred gain on the sale of building	9,128			8,166
Total deferred tax assets, prior to valuation allowance	85,203			70,989

Valuation allowance	(9,347		)	(8,557		)
Total deferred tax assets, net of valuation allowance	75,856			62,432

Deferred tax liabilities:
Prepaids	(1,335		)	(1,753		)
Depreciation	(13,047		)	(13,045		)
Intangibles	(56,952		)	(58,747		)
Total deferred tax liabilities	(71,334		)	(73,545		)

Net deferred tax assets (liabilities)	$	4,522		$	(11,113	)

As of December 31, 2015 and 2016, a valuation allowance has been established for certain deferred tax assets due to the uncertainty of realization. The valuation allowance as of December 31, 2015 and 2016 includes an allowance for unrealized losses on ARS investments, foreign deferred tax assets and state net operating losses and tax credits. The valuation allowance for the deferred tax asset for unrealized losses on ARS has been recorded as an adjustment to accumulated other comprehensive loss.

The Company established the valuation allowance because it is more likely than not that a portion of the deferred tax asset for certain items will not be realized based on the weight of available evidence. A valuation allowance was established for the unrealized losses on securities as the Company has not historically generated capital gains, and it is uncertain whether the Company will generate sufficient capital gains in the future to absorb the capital losses. A valuation allowance was established for the foreign deferred tax assets due to the cumulative loss in recent years in those jurisdictions. The Company has not had sufficient taxable income historically to utilize the foreign deferred tax assets, and it is uncertain whether the Company will generate sufficient taxable income in the future to utilize the deferred tax assets. Similarly, the Company has established a valuation allowance for net operating losses and tax credits in certain states where it is uncertain whether the Company will generate sufficient taxable income to utilize the net operating losses and tax credits before they expire.

The Company’s change in valuation allowance was an increase of approximately $2 million for the year ended December 31, 2015 and a decrease of approximately $1 million for the year ended December 31, 2016. The increase for the year ended December 31, 2015 is due to an increase in the valuation allowance for U.S. deferred tax assets of approximately $2 million primarily related to the change in local tax law that occurred during the first quarter of 2015, partially offset by the decrease in the valuation allowance for foreign deferred tax assets of approximately $551,000. The decrease for the year ended December 31, 2016 is due to a decrease in the valuation allowance for foreign deferred tax assets of approximately $1 million.

The Company had U.S. income before income taxes of approximately $71 million, $2 million and $135 million for the years ended December 31, 2014, 2015 and 2016, respectively. The Company had foreign income before income taxes of approximately $273,000, $1 million and $2 million for the years ended December 31, 2014, 2015 and 2016, respectively.


10.	INCOME TAXES — (CONTINUED)

The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands):



	Year Ended December 31,
	2014			2015			2016
Expected federal income tax provision at statutory rate	$	24,820		$	903		$	47,832
State income taxes, net of federal benefit	1,965			(678		)	3,638
Foreign income taxes, net effect	336			469			(31		)
Increase (decrease) in valuation allowance	(2,397		)	1,956			(103		)
Nondeductible compensation	554			574			141
Nondeductible transaction costs	—			229			103
Meals and entertainment	415			1,032			712
Tax rate changes	61			1,203			283
Research credits	—			—			(920		)
Other adjustments	290			358			(64		)
Income tax expense, net	$	26,044		$	6,046		$	51,591

The Company’s U.K. subsidiaries with foreign losses are disregarded entities for U.S. income tax purposes. Accordingly, the losses from these disregarded entities are included in the Company’s consolidated federal income tax provision at the statutory rate. Federal income taxes attributable to income from these disregarded entities are reduced by foreign taxes paid by those disregarded entities.

The Company paid approximately $3 million, $1 million, and $34 million in income taxes for the years ended December 31, 2014, 2015 and 2016, respectively.

The Company has net operating loss carryforwards for international income tax purposes of approximately $27 million, which do not expire. The Company has federal net operating loss carryforwards of approximately $49 million that begin to expire in 2020, state net operating loss carryforwards with a tax value of approximately $6 million that begin to expire in 2020 and state income tax credit carryforwards with a tax value of approximately $3 million that begin to expire in 2020. The Company realized a cash benefit relating to the use of its tax loss carryforwards of approximately $1 million, $1 million and $5 million in 2014, 2015 and 2016, respectively.

The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands):



Unrecognized tax benefits as of December 31, 2013	$	5,957
Increase for current year tax positions	51
Decrease for prior year tax positions	(189		)
Expiration of the statute of limitation for assessment of taxes	(70		)
Unrecognized tax benefits as of December 31, 2014	5,749
Increase for prior year tax positions	1,954
Expiration of the statute of limitation for assessment of taxes	(39		)
Unrecognized tax benefits as of December 31, 2015	7,664
Increase for current year tax positions	368
Decrease for prior year tax positions	(6,115		)
Expiration of the statute of limitation for assessment of taxes	(74		)
Unrecognized tax benefits as of December 31, 2016	$	1,843


10.	INCOME TAXES — (CONTINUED)

Approximately $1 million of the unrecognized tax benefits as of December 31, 2015 and 2016 would favorably affect the annual effective tax rate, if recognized in future periods. The Company recognized $62,000 and $83,000 for interest and penalties in its consolidated statements of operations for the years ended December 31, 2014 and 2015, respectively. The Company reversed interest and penalties of $416,000 in its consolidated statements of operations for the year ended December 31, 2016. The Company had liabilities of $466,000, $549,000 and $133,000 for interest and penalties in its consolidated balance sheets as of December 31, 2014, 2015 and 2016, respectively. The Company does not anticipate the amount of the unrecognized tax benefits will change significantly over the next twelve months.

The Company is subject to taxation in the U.S. and various states and foreign jurisdictions. The Company’s federal income tax returns and most state income tax returns for tax years 2013 through 2015 remain open to examination. For states that have a four-year statute of limitations, the state income tax returns for tax years 2012 through 2015 remain open to examination. The Company’s U.K. income tax returns for tax years 2010 through 2015 remain open to examination.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

The Company leases office facilities and office equipment under various non-cancelable operating leases. The leases contain various renewal options. Rent expense for the years ended December 31, 2014, 2015 and 2016 was approximately $19 million, $21 million and $22 million, respectively.

Future minimum lease payments as of December 31, 2016 are as follows (in thousands):



2017	$	26,548
2018	27,458
2019	24,969
2020	23,555
2021	22,337
2022 and thereafter	61,623
Total future minimum lease payments	$	186,490

On April 1, 2014, the Company entered into the 2014 Credit Agreement. The 2014 Credit Agreement provides for a $400 million term loan facility and a $225 million revolving credit facility, each with a term of five years. See Note 9 for additional information regarding the term loan facility and revolving credit facility.

Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated. At the present time, while it is reasonably possible that an unfavorable outcome may occur as a result of one or more of the Company’s current litigation matters, management has concluded that it is not probable that a loss has been incurred in connection with the Company’s current litigation. In addition, the Company is unable to estimate the possible loss or range of loss that could result from an unfavorable outcome in the Company’s current litigation and accordingly, the Company has not recognized any liability in the consolidated financial statements for unfavorable results, if any. Legal defense costs are expensed as incurred.

SEGMENT REPORTING

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

SEGMENT REPORTING

Segment Information

The Company manages its business geographically in two operating segments, with the primary areas of measurement and decision-making being North America, which includes the U.S. and Canada, and International, which includes the U.K., Spain, Germany and France. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. The Company’s subscription-based information services consist primarily of CoStar Suite®services. CoStar Suite is sold as a platform of service offerings consisting of CoStar Property Professional®, CoStar COMPS Professional® and CoStar Tenant® and through the Company's mobile application, CoStar Go®. CoStar Suite is the Company’s primary service offering in the North America and International operating segments. Management relies on an internal management reporting process that provides revenue and operating segment net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”). Management believes that operating segment EBITDA is an appropriate measure for evaluating the operational performance of the Company’s operating segments. EBITDA is used by management to internally measure operating and management performance and to evaluate the performance of the business. However, this measure should be considered in addition to, not as a substitute for or superior to, income from operations or other measures of financial performance prepared in accordance with GAAP.

Summarized information by operating segment consists of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Revenues
North America	$	552,141		$	686,573		$	809,492
International
External customers	23,795			25,191			28,138
Intersegment revenue	57			41			40
Total International revenue	23,852			25,232			28,178
Intersegment eliminations	(57		)	(41		)	(40		)
Total revenues	$	575,936		$	711,764		$	837,630

EBITDA
North America	$	148,913		$	87,092		$	210,901
International	2,337			2,895			4,169
Total EBITDA	$	151,250		$	89,987		$	215,070

The reconciliation of net income (loss) to EBITDA consists of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Net income (loss)	$	44,869		$	(3,465	)	$	85,071
Amortization of acquired intangible assets in cost of revenues	26,290			30,077			22,819
Amortization of acquired intangible assets in operating expenses	28,432			27,931			22,731
Depreciation and other amortization	15,650			20,524			24,615
Interest and other income	(516		)	(537		)	(1,773		)
Interest and other expense	10,481			9,411			10,016
Income tax expense, net	26,044			6,046			51,591
EBITDA	$	151,250		$	89,987		$	215,070


12.	SEGMENT REPORTING — (CONTINUED)

Segment Information — (Continued)

Intersegment revenues were attributable to services performed for the Company's wholly owned subsidiary, CoStar Portfolio Strategy by Grecam S.A.S. (“Grecam”), a wholly owned subsidiary of CoStar Limited, the Company’s wholly owned U.K. holding company. Intersegment revenues are recorded at an amount the Company believes approximates fair value. North America EBITDA includes a corresponding cost for the services performed by Grecam.

North America EBITDA includes an allocation of approximately $1 million, $954,000 and $396,000 for the years ended December 31, 2014, 2015 and 2016, respectively. This allocation represents costs incurred for International employees involved in development activities of the Company’s North America operating segment.

International EBITDA includes a corporate allocation of approximately $261,000, $256,000 and $321,000 for the years ended December 31, 2014, 2015 and 2016, respectively. This corporate allocation represents costs incurred for North America employees involved in management and expansion activities of the Company’s International operating segment.

Summarized information by operating segment consists of the following (in thousands):



	December 31,
	2015			2016
Property and equipment, net
North America	$	86,191		$	84,727
International	2,120			2,841
Total property and equipment, net	$	88,311		$	87,568

Goodwill
North America	$	1,227,310		$	1,227,777
International	25,635			27,089
Total goodwill	$	1,252,945		$	1,254,866

Assets
North America	$	2,130,202		$	2,239,587
International	41,370			45,167
Total operating segment assets	$	2,171,572		$	2,284,754

Reconciliation of operating segment assets to total assets
Total operating segment assets	$	2,171,572		$	2,284,754
Investment in subsidiaries	(18,344		)	(57,065		)
Intersegment receivables	(73,657		)	(42,626		)
Total assets	$	2,079,571		$	2,185,063

Liabilities
North America	$	525,566		$	520,833
International	72,544			50,057
Total operating segment liabilities	$	598,110		$	570,890

Reconciliation of operating segment liabilities to total liabilities
Total operating segment liabilities	$	598,110		$	570,890
Intersegment payables	(62,319		)	(40,040		)
Total liabilities	$	535,791		$	530,850


12.	SEGMENT REPORTING — (CONTINUED)

Revenues by Services

The Company provides information, analytics and online marketplaces to the commercial real estate industry. The revenues by type of service consists of the following (in thousands):



	Year Ended December 31,
	2014		2015		2016
Information and analytics
CoStar Suite⁽¹⁾	$	322,696	$	360,440	$	408,456
Information services⁽²⁾	72,525		75,790		77,178
Online marketplaces
Multifamily⁽³⁾	76,785		160,630		224,835
Commercial property and land⁽⁴⁾	103,930		114,904		127,161
Total revenues	$	575,936	$	711,764	$	837,630

⁽¹⁾ CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; CoStar Market Analytics; and CoStar Portfolio Strategy.

⁽²⁾ Information services is comprised of: LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex; and Thomas Daily.

⁽³⁾ Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com.

⁽⁴⁾ Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica and LandAndFarm; and CoStar Private Sale Network.

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 13: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

STOCKHOLDERS' EQUITY	12 Months Ended
STOCKHOLDERS' EQUITY	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS’ EQUITY Preferred Stock The Company has 2,000,000 shares of preferred stock, $0.01 par value, authorized for issuance as of December 31, 2016. The Board of Directors may issue the preferred stock from time to time as shares of one or more classes or series. Common Stock The Company has 60,000,000 shares of common stock, $0.01 par value, authorized for issuance. Dividends may be declared and paid on the common stock, subject in all cases to the rights and preferences of the holders of preferred stock and authorization by the Board of Directors. In the event of liquidation or winding up of the Company and after the payment of all preferential amounts required to be paid to the holders of any series of preferred stock, any remaining funds shall be distributed among the holders of the issued and outstanding common stock. Equity Offering During June 2014, the Company completed a public equity offering of 3,450,000 shares of common stock for $160.00 per share. Net proceeds from the public equity offering were approximately $529 million, after deducting approximately $22 million of underwriting discounts and commissions and offering expenses of approximately $500,000. The Company has used and intends to continue to use the net proceeds from the sale of the securities to fund all or a portion of the costs of any strategic acquisitions it determines to pursue, to finance the growth of its business and for general corporate purposes. General corporate purposes may include additions to working capital, capital expenditures, repayment of debt, investments in the Company’s subsidiaries, possible acquisitions and the repurchase, redemption or retirement of securities, including the Company’s common stock.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

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Reference 14: http://www.xbrl.org/2003/role/presentationRef
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+ Details

NET INCOME (LOSS) PER SHARE

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

NET INCOME (LOSS) PER SHARE

The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data):



	Year Ended December 31,
	2014		2015			2016
Numerator:
Net income (loss)	$	44,869	$	(3,465	)	$	85,071
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares	30,215		31,950			32,167
Effect of dilutive securities:
Stock options and restricted stock	426		—			269
Denominator for diluted net income (loss) per share — weighted-average outstanding shares	30,641		31,950			32,436

Net income (loss) per share — basic	$	1.48	$	(0.11	)	$	2.64
Net income (loss) per share — diluted	$	1.46	$	(0.11	)	$	2.62

Stock options to purchase approximately 80,000 and 194,000 shares that were outstanding for the years ended December 31, 2014 and 2016, respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Shares underlying restricted stock units that vest based on Company service conditions, that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands):



	Year Ended December 31,
	2014	2015	2016
Performance-based restricted stock awards	23	55	59
Service-based restricted stock units	1	1	1
Total shares excluded from computation	24	56	60

EMPLOYEE BENEFIT PLANS

12 Months Ended

Dec. 31, 2016

EMPLOYEE BENEFIT PLANS

Stock Incentive Plans

In April 2007, the Company’s Board of Directors adopted the CoStar Group, Inc. 2007 Stock Incentive Plan (as amended, the “2007 Plan”), subject to stockholder approval, which was obtained on June 7, 2007. In April 2016, the Company’s Board of Directors adopted the CoStar Group, Inc. 2016 Stock Incentive Plan (as amended, the “2016 Plan”), subject to stockholder approval, which was obtained on June 9, 2016. All shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (excluding shares subject to outstanding awards) were rolled into the 2016 Plan and, as of that date, no shares of common stock were available for new awards under the 2007 Plan. The 2007 Plan continues to govern unexercised and unexpired awards issued under the 2007 Plan prior to June 9, 2016. The 2007 Plan provided for the grant of stock options, restricted stock, restricted stock units and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2007 Plan could be incentive or non-qualified, and except in limited circumstances related to a merger or other acquisition, the exercise price for a stock option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period of the options, restricted stock and restricted stock unit grants under the 2007 Plan was determined by the Board of Directors or a committee thereof and was generally three to four years. In some cases, vesting of restricted stock awards under the 2007 Plan are subject to performance conditions. Upon the occurrence of a Change of Control, as defined in the 2007 Plan, all outstanding unexercisable options and restricted stock grants under the 2007 Plan immediately become exercisable.

The 2016 Plan provides for the grant of stock options, restricted stock, restricted stock units, and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2016 Plan may be non-qualified or may qualify as incentive stock options. Except in limited circumstances related to a merger or other acquisition, the exercise price for an option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period for each grant of options, restricted stock, restricted stock units and stock appreciation rights under the 2016 Plan is determined by the Board of Directors or a committee thereof and is generally three to four years, subject to minimum vesting periods for restricted stock and restricted stock units of at least one year. In some cases, vesting of awards under the 2016 Plan may be based on performance conditions. The Company has issued and/or reserved the following shares of common stock for issuance under the 2016 Plan: (a) 1,450,000 shares of common stock, plus (b) 815,464 shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (not including any Shares that were subject as of such date to outstanding awards under the 2007 Plan), and (c) any shares of common stock subject to outstanding awards under the 2007 Plan as of June 9, 2016, that on or after such date cease for any reason to be subject to such awards (other than by reason of exercise or settlement of the awards to the extent they are exercised for or settled in vested and nonforfeitable shares). Unless terminated sooner, the 2016 Plan will terminate in June 2026, but will continue to govern unexercised and unexpired awards issued under the 2016 Plan prior to that date. Approximately 2 million shares were available for future grant under the 2016 Plan as of December 31, 2016.

At December 31, 2016, there was approximately $57 million of unrecognized compensation cost related to stock incentive plans, net of forfeitures, which the Company expects to recognize over a weighted-average-period of 2.4 years.


15.	EMPLOYEE BENEFIT PLANS — (CONTINUED)

Stock Options

Option activity was as follows:



	Number of Shares		Range of Exercise Price	Weighted- Average Exercise Price		Weighted- Average Remaining Contract Life (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2013	374,340		$36.48 - $102.16	$	68.94
Granted	87,700		$201.04 - $201.04	$	201.04
Exercised	(68,126	)	$39.00 - $102.16	$	55.81
Canceled or expired	(23,735	)	$58.95 - $201.04	$	124.09
Outstanding at December 31, 2014	370,179		$36.48 - $201.04	$	99.12
Granted	89,500		$193.69 - $193.69	$	193.69
Exercised	(59,602	)	$36.48 - $201.04	$	85.48
Outstanding at December 31, 2015	400,077		$36.48 - $201.04	$	122.30
Granted	82,400		$182.75 - $182.75	$	182.75
Exercised	(29,285	)	$36.48 - $201.04	$	112.78
Canceled or expired	(13,034	)	$193.69 - $201.04	$	195.78
Outstanding at December 31, 2016	440,158		$36.48 - $201.04	$	132.08	6.54	$	26,086

Exercisable at December 31, 2014	188,656		$36.48 - $102.16	$	60.54
Exercisable at December 31, 2015	220,107		$36.48 - $201.04	$	77.63
Exercisable at December 31, 2016	284,489		$36.48 - $201.04	$	100.94	5.44	$	25,613

The aggregate intrinsic value is calculated as the difference between (i) the closing price of the common stock at the end of the period and (ii) the exercise prices of the underlying awards, multiplied by the shares underlying options as of the end of the period that had an exercise price less than the closing price on that date. Options to purchase 68,126, 59,602 and 29,285 shares were exercised during the years ended December 31, 2014, 2015, and 2016, respectively. The aggregate intrinsic value of options exercised, determined as of the date of option exercise, was approximately $9 million, $7 million and $3 million for the years ended December 31, 2014, 2015, and 2016, respectively.

The weighted-average grant date fair value of each option granted during the years ended December 31, 2014, 2015 and 2016 using the Black-Scholes option-pricing model was $58.12, $56.53 and $54.34, respectively.

The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table:



	Year Ended December 31,
	2014		2015		2016
Dividend yield	0	%	0	%	0	%
Expected volatility	30	%	30	%	31	%
Risk-free interest rate	2	%	2	%	1	%
Expected life (in years)	5		5		5


15.	EMPLOYEE BENEFIT PLANS — (CONTINUED)

Stock Options — (Continued)

The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the Company's common stock over a period consistent with the expected life of the options granted. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the options granted. The expected life for the options is determined based on multiple factors, including historical employee behavior patterns of exercising options and post-employment termination behavior as well as expected future employee option exercise patterns.

The following table summarizes information regarding options outstanding at December 31, 2016:



	Options Outstanding				Options Exercisable
Range of Exercise Price	Number of Shares	Weighted-Average Remaining Contractual Life (in years)	Weighted- Average Exercise Price		Number of Shares	Weighted- Average Exercise Price
$36.48 - $42.50	54,885	3.06	$	41.26	54,885	$	41.26
$42.51 - $58.06	42,156	4.14	$	56.53	42,156	$	56.53
$58.07 - $59.59	42,175	5.14	$	58.95	42,175	$	58.95
$59.60 - $81.19	2,320	4.42	$	60.23	2,320	$	60.23
$81.20 - $142.45	71,922	6.19	$	102.16	71,922	$	102.16
$142.46 - $188.22	82,400	9.19	$	182.75	—	$	—
$188.23 - $197.37	75,500	8.17	$	193.69	25,165	$	193.69
$197.38 - $201.04	68,800	7.16	$	201.04	45,866	$	201.04
$36.48 - $201.04	440,158	6.54	$	132.08	284,489	$	100.94

Restricted Stock Awards

These grants of restricted common stock are subject to continuing employment requirements and to a market condition. The actual number of shares that vest at the end of the respective three-year period is determined based on the Company’s achievement of the three-year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. At the end of the three-year performance period, if the performance condition is achieved at or above the pre-established threshold, the number of shares earned is further adjusted by a TSR payout percentage, which ranges between 80% and 120%, based on the Company’s TSR performance relative to that of the Russell 1000 Index over the respective three-year period. The Company granted a total of 24,720, 32,400 and 25,680 shares of performance-based restricted common stock during the years ended December 31, 2014, 2015 and 2016, respectively.


15.	EMPLOYEE BENEFIT PLANS — (CONTINUED)

Restricted Stock Awards — (Continued)

The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures. The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows:



	Year Ended December 31,
	2014			2015			2016
Dividend yield	0		%	0		%	0		%
Expected volatility	30		%	26		%	28		%
Risk-free interest rate	1		%	1		%	1		%
Expected life (in years)	3			3			3
Weighted-average grant date fair value	$	216.20		$	208.08		$	184.97

The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the common stock of the companies within the Russell 1000 Index over a period consistent with the expected life of the performance-based restricted common stock awards with a market condition. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the performance-based restricted common stock awards with a market condition. The expected life is consistent with the performance measurement period of the performance-based restricted common stock awards with a market condition.

As of December 31, 2016, the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2014, 2015 and 2016 performance-based restricted common stock awards would be met by their forfeiture dates. As a result, the Company recorded a total of approximately $1 million $3 million and $3 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2014, 2015 and 2016, respectively. The Company expects to record an aggregate amount of stock-based compensation expense related to the performance-based restricted common stock awards of approximately $5 million over the periods 2017, 2018 and 2019.

The following table presents unvested restricted stock awards activity for the year ended December 31, 2016:



	Restricted Stock Awards — without Market Condition				Restricted Stock Awards — with Market Condition
	Number of Shares		Weighted-Average Grant Date Fair Value per Share		Number of Shares		Weighted-Average Grant Date Fair Value per Share
Unvested restricted stock awards at December 31, 2015	503,113		$	166.42	54,960		$	211.41
Granted	170,710		$	194.77	25,680		$	184.97
Vested	(227,814	)	$	140.14	—		$	—
Canceled	(51,297	)	$	177.72	(8,160	)	$	211.18
Unvested restricted stock awards at December 31, 2016	394,712		$	192.38	72,480		$	202.07


15.	EMPLOYEE BENEFIT PLANS — (CONTINUED)

Restricted Stock Units

The following table presents unvested restricted stock units activity for the year ended December 31, 2016:



	Number of Shares		Weighted-Average Grant Date Fair Value per Share
Unvested restricted stock units at December 31, 2015	1,430		$	185.42
Granted	1,110		$	180.29
Vested	(371	)	$	177.93
Canceled	(970	)	$	185.85
Unvested restricted stock units at December 31, 2016	1,199		$	182.64

Employee 401(k) Plan

The Company maintains a 401(k) Plan (the “401(k)”) as a defined contribution retirement plan for all eligible employees. The 401(k) provides for tax-deferred contributions of employees’ salaries, limited to a maximum annual amount as established by the IRS. In addition to the traditional 401(k), effective January 1, 2015, eligible employees have the option of making an after-tax contribution to a Roth 401(k) plan or a combination of both. In 2014, 2015 and 2016, the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the 401(k) by the Company to match employee contributions for the years ended December 31, 2014, 2015 and 2016 were approximately $6 million, $8 million and $9 million, respectively. The Company had no administrative expenses in connection with the 401(k) plan for the years ended December 31, 2014, 2015 and 2016, respectively.

Employee Pension Plan

The Company maintains a Group Personal Pension Plan (the “Plan”) for all eligible employees in the Company’s U.K. offices. The Plan is a defined contribution plan. Employees are eligible to contribute a portion of their salaries, subject to a maximum annual amount as established by Her Majesty's Revenue and Customs. In 2014, 2015 and 2016, the Company's matching contribution was based on the percentage contributed by the employee, up to a maximum of 6% of total compensation. Amounts contributed to the Plan by the Company to match employee contributions for the years ended December 31, 2014, 2015 and 2016 were approximately $390,000, $420,000 and $380,000, respectively.

Registered Retirement Savings Plan

As of January 1, 2015, the Company introduced a registered retirement savings plan (“RRSP”) for all eligible employees in the Company’s Canadian offices. In 2015 and 2016, the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the RRSP by the Company to match employee contributions for the years ended December 31, 2015 and 2016 were approximately $40,000 and $10,000, respectively.

Employee Stock Purchase Plan

As of August 1, 2006, the Company introduced an Employee Stock Purchase Plan (“ESPP”), pursuant to which eligible employees participating in the plan authorize the Company to withhold specified amounts from the employees’ compensation and use the withheld amounts to purchase shares of the Company's common stock at 90% of the market price. Participating employees are able to purchase common stock under this plan during each offering period. An offering period begins the second Saturday before each of the Company’s regular pay dates and ends on each of the Company’s regular pay dates. On June 3, 2015, the Company’s stockholders approved an amendment to the ESPP to increase the number of shares available for purchase under the ESPP by 100,000 shares. On September 14, 2015, the Company registered the issuance of these additional shares under the ESPP pursuant to the registration statement filed September 14, 2015. There were 108,547 and 93,812 shares available for purchase under the ESPP as of December 31, 2015 and 2016, respectively, and approximately 13,227 and 14,735 shares of the Company’s common stock were purchased under the ESPP during 2015 and 2016, respectively.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Schedule II Valuation and Qualifying Accounts

12 Months Ended

Dec. 31, 2016

Valuation and Qualifying Accounts [Abstract]

Schedule II - Valuation and Qualifying Accounts

Schedule II – Valuation and Qualifying Accounts

Years Ended December 31, 2014, 2015, and 2016 (in thousands):



Allowance for Doubtful Accounts and Billing Adjustments ⁽¹⁾	Balance at Beginning of Year		Charged to Expense		Charged to Other Accounts ⁽²⁾		Write-offs, Net of Recoveries		Balance at End of Year
Year ended December 31, 2014	$	3,397	$	4,822	$	881	$	4,285	$	4,815
Year ended December 31, 2015	$	4,815	$	7,002	$	1,470	$	5,809	$	7,478
Year ended December 31, 2016	$	7,478	$	7,358	$	—	$	8,492	$	6,344


⁽¹⁾	Additions to the allowance for doubtful accounts are charged to bad debt expense.


⁽²⁾	Amounts represent opening balances from acquired businesses.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Revenue Recognition


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Revenue Recognition — (Continued)

Cost of Revenues

Foreign Currency Translation

Advertising Costs

The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising.

Income Taxes

Net Income (Loss) Per Share

Stock-Based Compensation

Cash and Cash Equivalents

Investments

Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis.

Concentration of Credit Risk and Financial Instruments

Accounts Receivable, Net of Allowance for Doubtful Accounts

Property and Equipment, Net



Leasehold improvements		Shorter of lease term or useful life
Furniture and office equipment		Five to ten years
Vehicles		Five to ten years
Computer hardware and software		Two to five years

Goodwill and Intangible Assets

Long-Lived Assets

Debt Issuance Costs

Business Combinations

Recent Accounting Pronouncements


2.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)

Recent Accounting Pronouncements — (Continued)

ACQUISITION (Policies)	12 Months Ended
ACQUISITION (Policies)	Dec. 31, 2016
Business Acquisition [Line Items]
Acquisitions	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings.
Apartment Finder [Member]
Business Acquisition [Line Items]
Acquisitions	The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date.

INVESTMENTS (Policies)	12 Months Ended
INVESTMENTS (Policies)	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities. Investments are carried at fair value. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value.

- Definition

Disclosure of accounting policy for investments in financial assets, including marketable securities (debt and equity securities with readily determinable fair values), investments accounted for under the equity method and cost method, securities borrowed and loaned, and repurchase and resale agreements. For marketable securities, the disclosure may include the entity's accounting treatment for transfers between investment categories and how the fair values for such securities are determined. Also, for all investments, an entity may describe its policy for assessing, recognizing and measuring impairment of the investment.

+ References + Details

COMMITMENTS AND CONTINGENCIES (Policies)	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Policies)	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated.

NET INCOME (LOSS) PER SHARE (Policies)	12 Months Ended
NET INCOME (LOSS) PER SHARE (Policies)	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Stock options to purchase approximately 80,000 and 194,000 shares that were outstanding for the years ended December 31, 2014 and 2016, respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Shares underlying restricted stock units that vest based on Company service conditions, that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share.

EMPLOYEE BENEFIT PLANS (Policies)	12 Months Ended
EMPLOYEE BENEFIT PLANS (Policies)	Dec. 31, 2016
Compensation Related Costs [Abstract]
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, employees, directors and consultants of the Company are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Schedule of accumulated other comprehensive loss

The components of accumulated other comprehensive loss were as follows (in thousands):



	As of December 31,
	2015			2016
Foreign currency translation adjustment	$	(7,159	)	$	(12,191	)
Net unrealized loss on investments, net of tax	(435		)	(848		)
Total accumulated other comprehensive loss	$	(7,594	)	$	(13,039	)

Stock-based compensation expense for stock options and restricted stock



	Year Ended December 31,
	2014		2015		2016
Cost of revenues	$	4,759	$	5,815	$	5,495
Selling and marketing	3,776		5,114		6,634
Software development	5,095		5,712		6,546
General and administrative	14,637		17,896		17,674
Total stock-based compensation	$	28,267	$	34,537	$	36,349

Schedule of property and equipment



Leasehold improvements		Shorter of lease term or useful life
Furniture and office equipment		Five to ten years
Vehicles		Five to ten years
Computer hardware and software		Two to five years

Property and equipment consists of the following (in thousands):



	December 31,
	2015			2016
Leasehold improvements	$	49,752		$	53,073
Furniture, office equipment and vehicles	38,129			45,035
Computer hardware and software	56,656			64,577
Property and equipment, gross	144,537			162,685
Accumulated depreciation and amortization	(56,226		)	(75,117		)
Property and equipment, net	$	88,311		$	87,568

ACQUISITION (Tables)

12 Months Ended

Dec. 31, 2016

Apartment Finder [Member]

Business Acquisition [Line Items]

Schedule of assets acquired and liabilities assumed

The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands):



Cash and cash equivalents	$	39
Accounts receivable	4,556
Goodwill	107,692
Acquired trade names and other intangible assets	23,642
Acquired customer base	21,856
Acquired database technology	4,076
Acquired building photography	2,425
Deferred income taxes, net	9,290
Other assets and liabilities	(849		)
Fair value of identifiable net assets acquired	$	172,727

INVESTMENTS (Tables)

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Scheduled maturities of investments classified as available-for-sale

Scheduled maturities of investments classified as available-for-sale as of December 31, 2016 are as follows (in thousands):



Maturity	Fair Value
Due in:
2017	$	—
2018 — 2021	—
2022 — 2026	—
2027 and thereafter	9,952
Available-for-sale investments	$	9,952

Schedule of available for sale securities reconciliation

As of December 31, 2016, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Auction rate securities	$	10,800	$	—	$	(848	)	$	9,952
Available-for-sale investments	$	10,800	$	—	$	(848	)	$	9,952

As of December 31, 2015, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Auction rate securities	$	15,942	$	610	$	(1,045	)	$	15,507
Available-for-sale investments	$	15,942	$	610	$	(1,045	)	$	15,507

Schedule of unrealized loss on investments

The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands):



	December 31,
	2015					2016
	Aggregate Fair Value		Gross Unrealized Losses			Aggregate Fair Value		Gross Unrealized Losses
Auction rate securities	$	14,455	$	(1,045	)	$	9,952	$	(848	)
Investments in an unrealized loss position	$	14,455	$	(1,045	)	$	9,952	$	(848	)

FAIR VALUE (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Summary of fair value hierarchy for the company's financial assets and liabilities measured at fair value on a recurring basis



	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	175,344	$	—	$	—	$	175,344
Commercial paper	6,383		—		—		6,383
Auction rate securities	—		—		9,952		9,952
Total assets measured at fair value	$	181,727	$	—	$	9,952	$	191,679



	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,043	$	—	$	—	$	5,043
Commercial paper	11,178		—		—		11,178
Auction rate securities	—		—		15,507		15,507
Total assets measured at fair value	$	16,221	$	—	$	15,507	$	31,728

Summary of changes in the fair value of the company's level 3 assets

The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2014 to December 31, 2016 (in thousands):



	Auction Rate Securities
Balance at December 31, 2014	$	17,151
Decrease in unrealized loss included in accumulated other comprehensive loss	256
Settlements	(1,900		)
Balance at December 31, 2015	15,507
Decrease in unrealized loss included in accumulated other comprehensive loss	395
Settlements	(5,950		)
Balance at December 31, 2016	$	9,952

PROPERTY AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Schedule of property and equipment



Leasehold improvements		Shorter of lease term or useful life
Furniture and office equipment		Five to ten years
Vehicles		Five to ten years
Computer hardware and software		Two to five years

Property and equipment consists of the following (in thousands):



	December 31,
	2015			2016
Leasehold improvements	$	49,752		$	53,073
Furniture, office equipment and vehicles	38,129			45,035
Computer hardware and software	56,656			64,577
Property and equipment, gross	144,537			162,685
Accumulated depreciation and amortization	(56,226		)	(75,117		)
Property and equipment, net	$	88,311		$	87,568

GOODWILL (Tables)

12 Months Ended

Dec. 31, 2016

Goodwill [Abstract]

Schedule of goodwill

The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands):



	North America		International			Total
Goodwill, December 31, 2014	$	1,114,363	$	24,442		$	1,138,805
Acquisition	112,947		2,400			115,347
Effect of foreign currency translation	—		(1,207		)	(1,207		)
Goodwill, December 31, 2015	1,227,310		25,635			1,252,945
Acquisitions	467		5,933			6,400
Effect of foreign currency translation	—		(4,479		)	(4,479		)
Goodwill, December 31, 2016	$	1,227,777	$	27,089		$	1,254,866

INTANGIBLE ASSETS (Tables)

12 Months Ended

Dec. 31, 2016

Intangible Assets, Net (Excluding Goodwill) [Abstract]

Schedule of acquired finite-lived intangible assets by major class

Intangible assets consist of the following (in thousands, except amortization period data):



	December 31,						Weighted- Average Amortization Period (in years)
	2015			2016			Weighted- Average Amortization Period (in years)
Capitalized product development cost	$	2,243		$	2,275		4
Accumulated amortization	(2,172		)	(2,217		)
Capitalized product development cost, net	71			58

Building photography	17,677			17,271			4
Accumulated amortization	(15,875		)	(16,351		)
Building photography, net	1,802			920

Acquired database technology	77,905			78,151			5
Accumulated amortization	(62,818		)	(72,691		)
Acquired database technology, net	15,087			5,460

Acquired customer base	221,409			220,749			10
Accumulated amortization	(129,782		)	(150,445		)
Acquired customer base, net	91,627			70,304

Acquired trade names and other intangible assets	153,910			153,607			13
Accumulated amortization	(24,179		)	(34,384		)
Acquired trade names and other intangible assets, net	129,731			119,223

Intangible assets, net	$	238,318		$	195,965

LONG-TERM DEBT (Tables)

12 Months Ended

Dec. 31, 2016

Long-term Debt, Current and Noncurrent [Abstract]

Components of long-term debt

The following table represents the Company's long-term debt (in thousands):



	December 31,
	2015			2016
Term loan facility	$	365,000		$	345,000
Debt issuance costs, net	(9,888		)	(6,661		)
Total debt	355,112			338,339
Current maturities of long-term debt	(20,000		)	(35,000		)
Current debt issuance costs, net	3,254			3,134
Total long-term debt, less current portion	$	338,366		$	306,473

Schedule of maturities of long-term debt

Maturities of the Company's borrowings under the 2014 Credit Agreement for each of the next three years as of December 31, 2016 are as follows (in thousands):



Year ending December 31,	Maturities
Due in:
2017	$	35,000
2018	55,000
2019	255,000
Long-term debt, including current maturities	$	345,000

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

+ Details

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Schedule of components of income tax expense

The components of the provision for income taxes attributable to operations consist of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Current:
Federal	$	24,741		$	10,295		$	32,198
State	2,761			1,503			3,682
Foreign	53			40			76
Total current	27,555			11,838			35,956
Deferred:
Federal	(698		)	(7,475		)	13,641
State	(813		)	1,683			1,959
Foreign	—			—			35
Total deferred	(1,511		)	(5,792		)	15,635
Total provision for income taxes	$	26,044		$	6,046		$	51,591

Schedule of deferred tax assets and liabilities

The components of deferred tax assets and liabilities consist of the following (in thousands):



	December 31,
	2015			2016
Deferred tax assets:
Reserve for bad debts	$	2,849		$	2,437
Accrued compensation	10,725			5,562
Stock compensation	12,686			14,268
Net operating losses	36,406			30,319
Accrued reserve and other	3,515			2,097
Unrealized loss on securities	377			326
Deferred rent	7,274			7,814
Deferred revenue	2,243			—
Deferred gain on the sale of building	9,128			8,166
Total deferred tax assets, prior to valuation allowance	85,203			70,989

Valuation allowance	(9,347		)	(8,557		)
Total deferred tax assets, net of valuation allowance	75,856			62,432

Deferred tax liabilities:
Prepaids	(1,335		)	(1,753		)
Depreciation	(13,047		)	(13,045		)
Intangibles	(56,952		)	(58,747		)
Total deferred tax liabilities	(71,334		)	(73,545		)

Net deferred tax assets (liabilities)	$	4,522		$	(11,113	)

Schedule of effective income tax rate reconciliation


10.	INCOME TAXES — (CONTINUED)

The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands):



	Year Ended December 31,
	2014			2015			2016
Expected federal income tax provision at statutory rate	$	24,820		$	903		$	47,832
State income taxes, net of federal benefit	1,965			(678		)	3,638
Foreign income taxes, net effect	336			469			(31		)
Increase (decrease) in valuation allowance	(2,397		)	1,956			(103		)
Nondeductible compensation	554			574			141
Nondeductible transaction costs	—			229			103
Meals and entertainment	415			1,032			712
Tax rate changes	61			1,203			283
Research credits	—			—			(920		)
Other adjustments	290			358			(64		)
Income tax expense, net	$	26,044		$	6,046		$	51,591

Schedule of unrecognized tax benefits

The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands):



Unrecognized tax benefits as of December 31, 2013	$	5,957
Increase for current year tax positions	51
Decrease for prior year tax positions	(189		)
Expiration of the statute of limitation for assessment of taxes	(70		)
Unrecognized tax benefits as of December 31, 2014	5,749
Increase for prior year tax positions	1,954
Expiration of the statute of limitation for assessment of taxes	(39		)
Unrecognized tax benefits as of December 31, 2015	7,664
Increase for current year tax positions	368
Decrease for prior year tax positions	(6,115		)
Expiration of the statute of limitation for assessment of taxes	(74		)
Unrecognized tax benefits as of December 31, 2016	$	1,843

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Future minimum lease payments

Future minimum lease payments as of December 31, 2016 are as follows (in thousands):



2017	$	26,548
2018	27,458
2019	24,969
2020	23,555
2021	22,337
2022 and thereafter	61,623
Total future minimum lease payments	$	186,490

SEGMENT REPORTING (Tables)

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

Summarized information by operating segment

Summarized information by operating segment consists of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Revenues
North America	$	552,141		$	686,573		$	809,492
International
External customers	23,795			25,191			28,138
Intersegment revenue	57			41			40
Total International revenue	23,852			25,232			28,178
Intersegment eliminations	(57		)	(41		)	(40		)
Total revenues	$	575,936		$	711,764		$	837,630

EBITDA
North America	$	148,913		$	87,092		$	210,901
International	2,337			2,895			4,169
Total EBITDA	$	151,250		$	89,987		$	215,070

Reconciliation of net income (loss) to EBITDA

The reconciliation of net income (loss) to EBITDA consists of the following (in thousands):



	Year Ended December 31,
	2014			2015			2016
Net income (loss)	$	44,869		$	(3,465	)	$	85,071
Amortization of acquired intangible assets in cost of revenues	26,290			30,077			22,819
Amortization of acquired intangible assets in operating expenses	28,432			27,931			22,731
Depreciation and other amortization	15,650			20,524			24,615
Interest and other income	(516		)	(537		)	(1,773		)
Interest and other expense	10,481			9,411			10,016
Income tax expense, net	26,044			6,046			51,591
EBITDA	$	151,250		$	89,987		$	215,070

Summarized information by operating segment, assets and liabilities

Summarized information by operating segment consists of the following (in thousands):



	December 31,
	2015			2016
Property and equipment, net
North America	$	86,191		$	84,727
International	2,120			2,841
Total property and equipment, net	$	88,311		$	87,568

Goodwill
North America	$	1,227,310		$	1,227,777
International	25,635			27,089
Total goodwill	$	1,252,945		$	1,254,866

Assets
North America	$	2,130,202		$	2,239,587
International	41,370			45,167
Total operating segment assets	$	2,171,572		$	2,284,754

Reconciliation of operating segment assets to total assets
Total operating segment assets	$	2,171,572		$	2,284,754
Investment in subsidiaries	(18,344		)	(57,065		)
Intersegment receivables	(73,657		)	(42,626		)
Total assets	$	2,079,571		$	2,185,063

Liabilities
North America	$	525,566		$	520,833
International	72,544			50,057
Total operating segment liabilities	$	598,110		$	570,890

Reconciliation of operating segment liabilities to total liabilities
Total operating segment liabilities	$	598,110		$	570,890
Intersegment payables	(62,319		)	(40,040		)
Total liabilities	$	535,791		$	530,850

Revenues by services

The revenues by type of service consists of the following (in thousands):



	Year Ended December 31,
	2014		2015		2016
Information and analytics
CoStar Suite⁽¹⁾	$	322,696	$	360,440	$	408,456
Information services⁽²⁾	72,525		75,790		77,178
Online marketplaces
Multifamily⁽³⁾	76,785		160,630		224,835
Commercial property and land⁽⁴⁾	103,930		114,904		127,161
Total revenues	$	575,936	$	711,764	$	837,630

⁽¹⁾ CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; CoStar Market Analytics; and CoStar Portfolio Strategy.

⁽³⁾ Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com.

⁽⁴⁾ Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica and LandAndFarm; and CoStar Private Sale Network.

NET INCOME (LOSS) PER SHARE (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Calculation of basic and diluted net income (loss) per share

The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data):



	Year Ended December 31,
	2014		2015			2016
Numerator:
Net income (loss)	$	44,869	$	(3,465	)	$	85,071
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares	30,215		31,950			32,167
Effect of dilutive securities:
Stock options and restricted stock	426		—			269
Denominator for diluted net income (loss) per share — weighted-average outstanding shares	30,641		31,950			32,436

Net income (loss) per share — basic	$	1.48	$	(0.11	)	$	2.64
Net income (loss) per share — diluted	$	1.46	$	(0.11	)	$	2.62

Schedule of anti-dilutive securities excluded from computation of earnings per share

The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands):



	Year Ended December 31,
	2014	2015	2016
Performance-based restricted stock awards	23	55	59
Service-based restricted stock units	1	1	1
Total shares excluded from computation	24	56	60

EMPLOYEE BENEFIT PLANS (Tables)

12 Months Ended

Dec. 31, 2016

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of option activity

Option activity was as follows:



	Number of Shares		Range of Exercise Price	Weighted- Average Exercise Price		Weighted- Average Remaining Contract Life (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2013	374,340		$36.48 - $102.16	$	68.94
Granted	87,700		$201.04 - $201.04	$	201.04
Exercised	(68,126	)	$39.00 - $102.16	$	55.81
Canceled or expired	(23,735	)	$58.95 - $201.04	$	124.09
Outstanding at December 31, 2014	370,179		$36.48 - $201.04	$	99.12
Granted	89,500		$193.69 - $193.69	$	193.69
Exercised	(59,602	)	$36.48 - $201.04	$	85.48
Outstanding at December 31, 2015	400,077		$36.48 - $201.04	$	122.30
Granted	82,400		$182.75 - $182.75	$	182.75
Exercised	(29,285	)	$36.48 - $201.04	$	112.78
Canceled or expired	(13,034	)	$193.69 - $201.04	$	195.78
Outstanding at December 31, 2016	440,158		$36.48 - $201.04	$	132.08	6.54	$	26,086

Exercisable at December 31, 2014	188,656		$36.48 - $102.16	$	60.54
Exercisable at December 31, 2015	220,107		$36.48 - $201.04	$	77.63
Exercisable at December 31, 2016	284,489		$36.48 - $201.04	$	100.94	5.44	$	25,613

Summarized information regarding options outstanding

The following table summarizes information regarding options outstanding at December 31, 2016:



	Options Outstanding				Options Exercisable
Range of Exercise Price	Number of Shares	Weighted-Average Remaining Contractual Life (in years)	Weighted- Average Exercise Price		Number of Shares	Weighted- Average Exercise Price
$36.48 - $42.50	54,885	3.06	$	41.26	54,885	$	41.26
$42.51 - $58.06	42,156	4.14	$	56.53	42,156	$	56.53
$58.07 - $59.59	42,175	5.14	$	58.95	42,175	$	58.95
$59.60 - $81.19	2,320	4.42	$	60.23	2,320	$	60.23
$81.20 - $142.45	71,922	6.19	$	102.16	71,922	$	102.16
$142.46 - $188.22	82,400	9.19	$	182.75	—	$	—
$188.23 - $197.37	75,500	8.17	$	193.69	25,165	$	193.69
$197.38 - $201.04	68,800	7.16	$	201.04	45,866	$	201.04
$36.48 - $201.04	440,158	6.54	$	132.08	284,489	$	100.94

Unvested restricted stock awards activity

The following table presents unvested restricted stock awards activity for the year ended December 31, 2016:



	Restricted Stock Awards — without Market Condition				Restricted Stock Awards — with Market Condition
	Number of Shares		Weighted-Average Grant Date Fair Value per Share		Number of Shares		Weighted-Average Grant Date Fair Value per Share
Unvested restricted stock awards at December 31, 2015	503,113		$	166.42	54,960		$	211.41
Granted	170,710		$	194.77	25,680		$	184.97
Vested	(227,814	)	$	140.14	—		$	—
Canceled	(51,297	)	$	177.72	(8,160	)	$	211.18
Unvested restricted stock awards at December 31, 2016	394,712		$	192.38	72,480		$	202.07

Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair value assumption for options granted

The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table:



	Year Ended December 31,
	2014		2015		2016
Dividend yield	0	%	0	%	0	%
Expected volatility	30	%	30	%	31	%
Risk-free interest rate	2	%	2	%	1	%
Expected life (in years)	5		5		5

Performance-based RSAs - with Market Condition [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair value assumption for options granted

The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows:



	Year Ended December 31,
	2014			2015			2016
Dividend yield	0		%	0		%	0		%
Expected volatility	30		%	26		%	28		%
Risk-free interest rate	1		%	1		%	1		%
Expected life (in years)	3			3			3
Weighted-average grant date fair value	$	216.20		$	208.08		$	184.97

Restricted Stock Units (RSUs) [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Unvested restricted stock awards activity

The following table presents unvested restricted stock units activity for the year ended December 31, 2016:



	Number of Shares		Weighted-Average Grant Date Fair Value per Share
Unvested restricted stock units at December 31, 2015	1,430		$	185.42
Granted	1,110		$	180.29
Vested	(371	)	$	177.93
Canceled	(970	)	$	185.85
Unvested restricted stock units at December 31, 2016	1,199		$	182.64

ORGANIZATION (Details)	12 Months Ended
ORGANIZATION (Details)	Dec. 31, 2016 operating_segments
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of business segments (in segments)	2
Term of subscription-based license agreements (in years)	1 year

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - revenue Recognition and Advertising Costs) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)	1 year
Advertising costs	$ 109	$ 132	$ 29

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Foreign Currency Translation and Accumulated Other Comprehensive Loss) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract]
Material gains or losses from foreign currency exchange transactions	$ 0	$ 0	$ 0
Foreign currency translation adjustment	(12,191,000)	(7,159,000)
Net unrealized loss on investments, net of tax	(848,000)	(435,000)
Total accumulated other comprehensive loss	(13,039,000)	(7,594,000)
Reclassifications out of accumulated other comprehensive loss	$ (808,000)	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Stock-Based Compensation) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 36,349	$ 34,537	$ 28,267
Performance service period	3 years
Net cash proceeds from the exercise of stock options and ESPP	$ 5,861	7,404	5,740
Excess tax benefit from stock-based compensation	4,698	8,528	28,406
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,495	5,815	4,759
Selling and marketing
Stock-Based Compensation Expense [Abstract]
Compensation expense	6,634	5,114	3,776
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	6,546	5,712	5,095
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	17,674	17,896	14,637
Performance-based RSAs - with Market Condition [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	3,000	$ 3,000	$ 1,000
Compensation cost expected to be recognized in future years	$ 5,000

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Cash and Cash Equivalents and Concentration of Credit Risk)	12 Months Ended
	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Number of customers accounting for more than 5% of revenue (in customers)	No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2014, 2015 and 2016.
Maximum original maturity of highly liquid investments to be considered cash equivalents	The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Property and Equipment)	12 Months Ended
	Dec. 31, 2016
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	Shorter of lease term or useful life
Furniture and Office Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Furniture and Office Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Vehicles [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Vehicles [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Computer Hardware and Software [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	2 years
Computer Hardware and Software [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Goodwill and Intangible Assets)	12 Months Ended
	Dec. 31, 2016
Acquired Database Technology [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Database Technology [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	8 years
Acquired Trade Names and Other Intangible Assets [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Trade Names and Other Intangible Assets [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years
Acquired Customer Base [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	10 years
Acquired Customer Base [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	13 years
Acquired Building Photography [Member] \| Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Building Photography [Member] \| Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Debt Issuance Costs and Recent Accounting Pronouncements) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Capitalized debt issuance costs, net	$ 6,661	$ 9,888
Amortization of debt issuance costs	$ 3,000	$ 3,000	$ 3,000

ACQUISITION (Details for Apartment Finder)		12 Months Ended
ACQUISITION (Details for Apartment Finder)	Jun. 01, 2015 USD ($) distinct_intangible_asset	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 1,254,866,000	$ 1,252,945,000	$ 1,138,805,000
Apartment Finder [Member]
Business Acquisition [Line Items]
Percentage of outstanding stock acquired	100.00%
Payments to acquire businesses	$ 173,000,000
Closing purchase price adjustments	3,000,000
Post closing purchase price adjustments	21,000
Goodwill	107,692,000
Goodwill tax deductible amount	$ 0
Acquired Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		13 years
Acquired Trade Names and Other Intangible Assets [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	9 years
Acquired Customer Base [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		10 years
Acquired Customer Base [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Number of distinct intangible assets \| distinct_intangible_asset	3
Estimated useful life of acquired assets	10 years
Acquired Database Technology [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		5 years
Acquired Database Technology [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months
Acquired Building Photography [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		4 years
Acquired Building Photography [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months

ACQUISITION ACQUISITION (Details - Schedule of Assets Acquired and Liabilities Assumed for Apartment Finder) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Jun. 01, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 1,254,866	$ 1,252,945		$ 1,138,805
Apartment Finder [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 39
Accounts receivable			4,556
Goodwill			107,692
Deferred income taxes, net			9,290
Other assets and liabilities			(849)
Fair value of identifiable net assets acquired			172,727
Acquired Trade Names and Other Intangible Assets [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			23,642
Acquired Customer Base [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			21,856
Acquired Database Technology [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			4,076
Acquired Building Photography [Member] \| Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			$ 2,425

INVESTMENTS, SCHEDULED MATURITIES AND REALIZED GAINS AND LOSSES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, gross realized gains	$ 808,000	$ 0	$ 0
Proceeds from sales of available-for-sale securities	1,000,000
Available-for-sale securities, gross realized losses	0	0	$ 0
Debt Maturities Fair Value [Abstract]
2017	0
2018 — 2021	0
2022 — 2026	0
2027 and thereafter	9,952,000
Available-for-sale investments	$ 9,952,000	$ 15,507,000

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	$ 10,800,000	$ 15,942,000
Gross unrealized gains	0	610,000
Gross unrealized losses	(848,000)	(1,045,000)
Fair Value	9,952,000	15,507,000
Available-for-sale securities, unrealized loss positions
Continuous unrealized loss position, 12 months or more, aggregated fair value	9,952,000	14,455,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	(848,000)	(1,045,000)
Continuous unrealized loss position, less than 12 months	0	0
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	10,800,000	15,942,000
Gross unrealized gains	0	610,000
Gross unrealized losses	(848,000)	(1,045,000)
Fair Value	9,952,000	15,507,000
Available-for-sale securities, unrealized loss positions
Continuous unrealized loss position, 12 months or more, aggregated fair value	9,952,000	14,455,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	$ (848,000)	$ (1,045,000)

FAIR VALUE (Details) - USD ($) $ in Thousands	12 Months Ended
FAIR VALUE (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Unobservable inputs assets (level 3) [Roll forward]
Temporary impairment of the auction rates security investments	$ (848)	$ (435)
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets measured at fair value	191,679	31,728
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	175,344	5,043
Fair Value, Measurements, Recurring [Member] \| Commercial Paper [Member]
Assets:
Total assets measured at fair value	6,383	11,178
Fair Value, Measurements, Recurring [Member] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	9,952	15,507
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	181,727	16,221
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	175,344	5,043
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Commercial Paper [Member]
Assets:
Total assets measured at fair value	6,383	11,178
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Assets:
Total assets measured at fair value	9,952	15,507
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	9,952	15,507
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	15,507	17,151
Decrease in unrealized loss included in accumulated other comprehensive loss	395	256
Settlements	(5,950)	(1,900)
Ending balance	$ 9,952	$ 15,507
Auction rate securities variable rate debt instruments interest rate reset period	28 days
The minimum contractual maturities on the underlying securities involved in the auction rate securities (in years)	20 years
Par value of company held auction rate securities	$ 11,000
Discount rate (in percent)	5.00%	5.00%
Temporary impairment of the auction rates security investments	$ (848)

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 162,685	$ 144,537
Accumulated depreciation and amortization	(75,117)	(56,226)
Property and equipment, net	87,568	88,311
Depreciation expense for property and equipment	24,000	20,000	$ 15,000
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	53,073	49,752
Furniture, Office Equipment and Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	45,035	38,129
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 64,577	$ 56,656

GOODWILL (Details) - USD ($) $ in Thousands				12 Months Ended
GOODWILL (Details) - USD ($) $ in Thousands	May 03, 2016	Jul. 01, 2015	Jun. 01, 2015	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Goodwill, beginning balance				$ 1,252,945	$ 1,138,805
Acquisitions				6,400	115,347
Effect of foreign currency translation				(4,479)	(1,207)
Goodwill, ending balance				1,254,866	1,252,945
Apartment Finder [Member]
Goodwill [Roll Forward]
Acquisitions			$ 108,000
Goodwill, ending balance			$ 107,692
Belbex [Member]
Goodwill [Roll Forward]
Acquisitions		$ 2,000
Independent Distributor Buyout [Member]
Goodwill [Roll Forward]
Acquisitions				467	5,000
Thomas Daily [Member]
Goodwill [Roll Forward]
Acquisitions	$ 6,000
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance				1,227,310	1,114,363
Acquisitions				467	112,947
Effect of foreign currency translation				0	0
Goodwill, ending balance				1,227,777	1,227,310
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance				25,635	24,442
Acquisitions				5,933	2,400
Effect of foreign currency translation				(4,479)	(1,207)
Goodwill, ending balance				$ 27,089	$ 25,635

INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	12 Months Ended
INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$ 195,965	$ 238,318
Amortization of intangible assets	46,000	59,000	$ 55,000
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization expense for 2017	33,000
Amortization expense for 2018	24,000
Amortization expense for 2019	21,000
Amortization expense for 2020	19,000
Amortization expense for 2021	18,000
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	2,275	2,243
Finite-lived intangible assets, accumulated amortization	(2,217)	(2,172)
Finite-lived intangible assets, net	$ 58	71
Weighted-average amortization period (in years}	4 years
Acquired Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 17,271	17,677
Finite-lived intangible assets, accumulated amortization	(16,351)	(15,875)
Finite-lived intangible assets, net	$ 920	1,802
Weighted-average amortization period (in years}	4 years
Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 78,151	77,905
Finite-lived intangible assets, accumulated amortization	(72,691)	(62,818)
Finite-lived intangible assets, net	$ 5,460	15,087
Weighted-average amortization period (in years}	5 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 220,749	221,409
Finite-lived intangible assets, accumulated amortization	(150,445)	(129,782)
Finite-lived intangible assets, net	$ 70,304	91,627
Weighted-average amortization period (in years}	10 years
Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 153,607	153,910
Finite-lived intangible assets, accumulated amortization	(34,384)	(24,179)
Finite-lived intangible assets, net	$ 119,223	$ 129,731
Weighted-average amortization period (in years}	13 years
LoopNet [Member] \| Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	15 years

LONG-TERM DEBT (Details) - USD ($)		1 Months Ended	12 Months Ended
LONG-TERM DEBT (Details) - USD ($)	Apr. 03, 2014	Jun. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2014
Business Acquisition [Line Items]
Repayments of term loan	$ 149,000,000
LIBOR period (in months)	1 month
Default interest rate per annum on overdue amounts (in percent)					2.00%
Credit facility, collateral			The obligations under the 2014 Credit Agreement are guaranteed by all material subsidiaries of the Company and are secured by a lien on substantially all of the assets of the Company and those of its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee documents entered into on the Closing Date.
Term Loan [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					$ 400,000,000
Term of loan (in years)	5 years
Revolving Credit Facility [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					225,000,000
Term of loan (in years)	5 years
Line of credit facility, amount outstanding			$ 0	$ 0	150,000,000
Repayments of revolving credit facility		$ 150,000,000
CoStar Group [Member] \| Swingline Loan [Member]
Business Acquisition [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit					10,000,000
CoStar Group [Member] \| Letter of Credit [Member]
Business Acquisition [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit					$ 10,000,000
CoStar Group [Member] \| Notes Payable to Banks [Member]
Business Acquisition [Line Items]
Annual amortization, first year after closing (in percent)					5.00%
Annual amortization, second year after closing (in percent)					5.00%
Annual amortization, third year after closing (in percent)					5.00%
Annual amortization, fourth year after closing (in percent)					10.00%
Annual amortization, fifth year after closing (in percent)					15.00%
Federal Funds Rate [Member]
Business Acquisition [Line Items]
Basis spread on federal funds rate (in percent)	0.50%
LIBOR [Member]
Business Acquisition [Line Items]
Basis spread on federal funds rate (in percent)	2.00%
Basis spread on variable rate, one month interest period (in percent)					1.00%
Basis spread on variable rate, per annum (in percent)					1.00%

LONG-TERM DEBT (Details - Covenant Calculations)	Apr. 01, 2014
Debt Disclosure [Abstract]
Maximum first lien secured leverage ratio after eight quarters after closing date	350.00%
Maximum total leverage ratio after eight fiscal quarters	450.00%

LONG-TERM DEBT (Details - Interest Expense and Amortization of Debt Issuance Costs) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Apr. 01, 2014
Debt Instrument [Line Items]
Capitalized debt issuance costs, net				$ 10,000,000
Interest expense, debt	$ 10,000,000	$ 9,000,000	$ 10,000,000
Amortization of debt issuance costs	3,000,000	3,000,000	3,000,000
Interest paid	7,000,000	6,000,000	$ 7,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 0	$ 0		$ 150,000,000

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References + Details

LONG-TERM DEBT (Details - Components of Long-Term Debt) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Term loan facility	$ 345,000	$ 365,000
Debt issuance costs, net	(6,661)	(9,888)
Total debt	338,339	355,112
Current maturities of long-term debt	(35,000)	(20,000)
Current debt issuance costs, net	3,134	3,254
Total long-term debt, less current portion	$ 306,473	$ 338,366

LONG-TERM DEBT (Details - Schedule of Maturities of Long-Term Debt) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Long-term Debt, Current and Noncurrent [Abstract]
2017	$ 35,000
2018	55,000
2019	255,000
Long-term debt, including current maturities	$ 345,000	$ 365,000

INCOME TAXES (Details - Components for Provision for Income Taxes) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 32,198	$ 10,295	$ 24,741
State	3,682	1,503	2,761
Foreign	76	40	53
Total current	35,956	11,838	27,555
Deferred:
Federal	13,641	(7,475)	(698)
State	1,959	1,683	(813)
Foreign	35	0	0
Total deferred	15,635	(5,792)	(1,511)
Income tax expense, net	$ 51,591	$ 6,046	$ 26,044

INCOME TAXES (Details - Components of Deferred Tax Assets and Liabilities) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Reserve for bad debts	$ 2,437	$ 2,849
Accrued compensation	5,562	10,725
Stock compensation	14,268	12,686
Net operating losses	30,319	36,406
Accrued reserve and other	2,097	3,515
Unrealized loss on securities	326	377
Deferred rent	7,814	7,274
Deferred revenue	0	2,243
Deferred gain on the sale of building	8,166	9,128
Total deferred tax assets, prior to valuation allowance	70,989	85,203
Valuation allowance	(8,557)	(9,347)
Total deferred tax assets, net of valuation allowance	62,432	75,856
Deferred tax liabilities:
Prepaids	(1,753)	(1,335)
Depreciation	(13,045)	(13,047)
Intangibles	(58,747)	(56,952)
Total deferred tax liabilities	(73,545)	(71,334)
Net deferred tax assets		$ 4,522
Net deferred tax liabilities	$ (11,113)

INCOME TAXES (Details - Reconciliation of Provision for Income Taxes) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective tax rate reconciliation [Abstract]
Expected federal income tax provision at statutory rate	$ 47,832	$ 903	$ 24,820
State income taxes, net of federal benefit	3,638	(678)	1,965
Foreign income taxes, net effect	(31)	469	336
Increase (decrease) in valuation allowance	(103)	1,956	(2,397)
Nondeductible compensation	141	574	554
Nondeductible transaction costs	103	229	0
Meals and entertainment	712	1,032	415
Tax rate changes	283	1,203	61
Research credits	(920)	0	0
Other adjustments	(64)	358	290
Income tax expense, net	$ 51,591	$ 6,046	$ 26,044

INCOME TAXES (Details - Other) - USD ($) $ in Thousands	12 Months Ended
INCOME TAXES (Details - Other) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ (1,000)	$ 2,000
Cash tax benefits resulting in net operating loss carryforward	5,000	1,000	$ 1,000
Income from U.S. sources	135,000	2,000	71,000
Income from foreign sources	2,000	1,000	273
Income taxes paid	34,000	1,000	$ 3,000
Foreign Country [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	(1,000)	(551)
Net operating loss carryforward	27,000
Domestic Country [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 49,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance		$ 2,000
Net operating loss carryforward	$ 6,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
Income tax credit carryforward	$ 3,000

INCOME TAXES (Details - Unrecognized Tax Benefits) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits beginning balance	$ 7,664,000	$ 5,749,000	$ 5,957,000
Increase for current year tax positions	368,000		51,000
Increase for prior year tax positions		1,954,000
Decrease for prior period tax positions	(6,115,000)		(189,000)
Expiration of the statute of limitation for assessment of taxes	(74,000)	(39,000)	(70,000)
Unrecognized tax benefits ending balance	1,843,000	7,664,000	5,749,000
Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods	1,000,000	1,000,000
Interest and penalties on income taxes recognized		83,000	62,000
Interest and penalties on income taxes reversed	416,000
Interest and penalties accrued on income taxes	$ 133,000	$ 549,000	$ 466,000

COMMITMENTS AND CONTINGENCIES (Details) - USD ($)		12 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	Apr. 03, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Apr. 01, 2014
Operating Leases Rent Expense [Abstract]
Rent expense for operating leases		$ 22,000,000	$ 21,000,000	$ 19,000,000
Future Minimum Lease Payments [Abstract]
2017		26,548,000
2018		27,458,000
2019		24,969,000
2020		23,555,000
2021		22,337,000
2022 and thereafter		61,623,000
Total future minimum lease payments		$ 186,490,000
Term Loan [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					$ 400,000,000
Term of loan (in years)	5 years
Revolving Credit Facility [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					$ 225,000,000
Term of loan (in years)	5 years

SEGMENT REPORTING (Details) $ in Thousands	12 Months Ended
SEGMENT REPORTING (Details) $ in Thousands	Dec. 31, 2016 USD ($) operating_segments	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of business segments (in segments) \| operating_segments	2
Segment Reporting Information, Revenue [Abstract]
Revenues	$ 837,630	$ 711,764	$ 575,936
Reconciliation of EBITDA to net income [Abstract]
Net income (loss)	85,071	(3,465)	44,869
Amortization of acquired intangible assets in cost of revenues	22,819	30,077	26,290
Amortization of acquired intangible assets in operating expenses	22,731	27,931	28,432
Depreciation and other amortization	24,615	20,524	15,650
Interest and other income	(1,773)	(537)	(516)
Interest and other expense	10,016	9,411	10,481
Income tax expense, net	51,591	6,046	26,044
EBITDA	215,070	89,987	151,250
International [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	28,178	25,232	23,852
Operating Segments [Member] \| North America [Member]
Segment Reporting Information [Line Items]
North America corporate allocation	396	954	1,000
Segment Reporting Information, Revenue [Abstract]
Revenues	809,492	686,573	552,141
Reconciliation of EBITDA to net income [Abstract]
EBITDA	210,901	87,092	148,913
Operating Segments [Member] \| International [Member]
Segment Reporting Information [Line Items]
International corporate allocation	321	256	261
Reconciliation of EBITDA to net income [Abstract]
EBITDA	4,169	2,895	2,337
Operating Segments [Member] \| International [Member] \| External Customers [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	28,138	25,191	23,795
Intersegment Revenue [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	40	41	57
Intersegment Eliminations [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	$ (40)	$ (41)	$ (57)

SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Property and equipment, net	$ 87,568	$ 88,311
Goodwill	1,254,866	1,252,945	$ 1,138,805
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,185,063	2,079,571
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	530,850	535,791
North America [Member]
Segment Reporting Information [Line Items]
Goodwill	1,227,777	1,227,310	1,114,363
International [Member]
Segment Reporting Information [Line Items]
Goodwill	27,089	25,635	$ 24,442
Operating Segments [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,284,754	2,171,572
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	570,890	598,110
Operating Segments [Member] \| North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	84,727	86,191
Goodwill	1,227,777	1,227,310
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,239,587	2,130,202
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	520,833	525,566
Operating Segments [Member] \| International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	2,841	2,120
Goodwill	27,089	25,635
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	45,167	41,370
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	50,057	72,544
Consolidation Eliminations [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	(57,065)	(18,344)
Intersegment Eliminations [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	(42,626)	(73,657)
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ (40,040)	$ (62,319)

SEGMENT REPORTING SEGMENT REPORTING, REVENUES BY SERVICES (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2016

Dec. 31, 2015

Dec. 31, 2014

Revenue from External Customer [Line Items]

Revenues

$ 837,630

$ 711,764

$ 575,936

CoStar Suite [Member]

Revenue from External Customer [Line Items]

Revenues

^[1]

408,456

360,440

322,696

Information services [Member]

Revenue from External Customer [Line Items]

Revenues

^[2]

77,178

75,790

72,525

Multifamily [Member]

Revenue from External Customer [Line Items]

Revenues

^[3]

224,835

160,630

76,785

Commercial property and land [Member]

Revenue from External Customer [Line Items]

Revenues

^[4]

$ 127,161

$ 114,904

$ 103,930

[1]	CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; CoStar Market Analytics; and CoStar Portfolio Strategy.
[2]	Information services is comprised of: LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex; and Thomas Daily.
[3]	Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com.
[4]	Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica and LandAndFarm; and CoStar Private Sale Network.

STOCKHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Preferred stock
Preferred stock authorized for issuance (in shares)		2,000,000	2,000,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock
Common stock authorized for issuance (in shares)		60,000,000	60,000,000
Common stock par value (in dollars per share)		$ 0.01	$ 0.01
Equity offering
Equity offering in period, new shares (in shares)	3,450,000
New shares issued, price per share (in dollars per share)	$ 160.00
Proceeds from equity offering, net of transaction costs	$ 529,000	$ 0	$ 0	$ 529,360
Payments for underwriting expense	22,000
Payments of equity offering costs	$ 500

NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income (loss)	$ 85,071	$ (3,465)	$ 44,869
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares	32,167	31,950	30,215
Effect of dilutive securities:
Stock options and restricted stock (in shares)	269	0	426
Denominator for diluted net income (loss) per share — weighted-average outstanding shares	32,436	31,950	30,641
Net income (loss) per share — basic (in dollars per share)	$ 2.64	$ (0.11)	$ 1.48
Net income (loss) per share — diluted (in dollars per share)	$ 2.62	$ (0.11)	$ 1.46
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	60	56	24
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	194		80
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	59	55	23
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	1	1	1

EMPLOYEE BENEFIT PLANS (Details for Stock Incentive Plans) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 09, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 36,349	$ 34,537	$ 28,267
Performance service period	3 years
Options Outstanding 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	$ 36.48
Range of exercise price, maximum, (in dollars per share)	$ 42.50
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	54,885
Weighted-average remaining contractual life (in years)	3 years 23 days
Weighted- average exercise price (in dollars per share)	$ 41.26
Options Outstanding 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	42.51
Range of exercise price, maximum, (in dollars per share)	$ 58.06
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	42,156
Weighted-average remaining contractual life (in years)	4 years 1 month 20 days
Weighted- average exercise price (in dollars per share)	$ 56.53
Options Outstanding 3 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	58.07
Range of exercise price, maximum, (in dollars per share)	$ 59.59
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	42,175
Weighted-average remaining contractual life (in years)	5 years 1 month 20 days
Weighted- average exercise price (in dollars per share)	$ 58.95
Options Outstanding 4 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	59.60
Range of exercise price, maximum, (in dollars per share)	$ 81.19
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	2,320
Weighted-average remaining contractual life (in years)	4 years 5 months
Weighted- average exercise price (in dollars per share)	$ 60.23
Options Outstanding 5 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	81.20
Range of exercise price, maximum, (in dollars per share)	$ 142.45
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	71,922
Weighted-average remaining contractual life (in years)	6 years 2 months 8 days
Weighted- average exercise price (in dollars per share)	$ 102.16
Options Outstanding 6 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	142.46
Range of exercise price, maximum, (in dollars per share)	$ 188.22
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	82,400
Weighted-average remaining contractual life (in years)	9 years 2 months 8 days
Weighted- average exercise price (in dollars per share)	$ 182.75
Options Outstanding 7 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	188.23
Range of exercise price, maximum, (in dollars per share)	$ 197.37
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	75,500
Weighted-average remaining contractual life (in years)	8 years 2 months 2 days
Weighted- average exercise price (in dollars per share)	$ 193.69
Options Outstanding 8 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	197.38
Range of exercise price, maximum, (in dollars per share)	$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	68,800
Weighted-average remaining contractual life (in years)	7 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 201.04
Options Outstanding 9 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	36.48
Range of exercise price, maximum, (in dollars per share)	$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	440,158
Weighted-average remaining contractual life (in years)	6 years 6 months 14 days
Weighted- average exercise price (in dollars per share)	$ 132.08
Options Exercisable 1 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	54,885
Weighted-average exercise price (in dollars per share)	$ 41.26
Options Exercisable 2 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	42,156
Weighted-average exercise price (in dollars per share)	$ 56.53
Options Exercisable 3 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	42,175
Weighted-average exercise price (in dollars per share)	$ 58.95
Options Exercisable 4 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	2,320
Weighted-average exercise price (in dollars per share)	$ 60.23
Options Exercisable 5 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	71,922
Weighted-average exercise price (in dollars per share)	$ 102.16
Options Exercisable 6 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	0
Weighted-average exercise price (in dollars per share)	$ 0.00
Options Exercisable 7 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	25,165
Weighted-average exercise price (in dollars per share)	$ 193.69
Options Exercisable 8 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	45,866
Weighted-average exercise price (in dollars per share)	$ 201.04
Options Exercisable 9 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	284,489
Weighted-average exercise price (in dollars per share)	$ 100.94
CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares)	2,000,000
Shares of common stock authorized for issuance under the plan (in shares)	1,450,000
CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares)				0
Shares of common stock authorized for issuance under the plan (in shares)	815,464
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	1,430
Granted (in shares)	1,110
Vested (in shares)	(371)
Canceled (in shares)	(970)
Unvested restricted stock at end of period (in shares)	1,199	1,430
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 185.42
Granted (in dollars per share)	180.29
Vested (in dollars per share)	177.93
Canceled (in dollars per share)	185.85
Unvested restricted stock at end of period (in dollars per share)	$ 182.64	$ 185.42
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost expected to be recognized in future years	$ 57,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 4 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)	400,077	370,179	374,340
Granted (in shares)	82,400	89,500	87,700
Exercised (in shares)	(29,285)	(59,602)	(68,126)
Canceled or expired (in shares)	(13,034)		(23,735)
Outstanding at end of period (in shares)	440,158	400,077	370,179
Exercisable at end of period (in shares)	284,489	220,107	188,656
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average exercise price, outstanding at beginning of period (in dollars per share)	$ 122.30	$ 99.12	$ 68.94
Weighted-average exercise price, granted (in dollars per share)	182.75	193.69	201.04
Weighted-average exercise price, exercised (in dollars per share)	112.78	85.48	55.81
Weighted-average exercise price, canceled or expired (in dollars per share)	195.78		124.09
Weighted-average exercise price, outstanding at end of period (in dollars per share)	132.08	122.30	99.12
Weighted-average exercise price, exercisable at end of period (in dollars per share)	$ 100.94	$ 77.63	$ 60.54
Weighted-average remaining contract life of options outstanding at end of period	6 years 6 months 15 days
Weighted-average remaining contract life of options exercisable at end of period	5 years 5 months 9 days
Aggregate intrinsic value of options outstanding at end of period	$ 26,086
Aggregate intrinsic value of options exercisable at end of period	25,613
Aggregate intrinsic value of options exercised	$ 3,000	$ 7,000	$ 9,000
Weighted-average grant date fair value of each option granted during the period (in dollars per share)	$ 54.34	$ 56.53	$ 58.12
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)	0.00%	0.00%	0.00%
Expected volatility (in hundredths)	31.00%	30.00%	30.00%
Risk-free interest rate (in hundredths)	1.00%	2.00%	2.00%
Expected life (in years)	5 years	5 years	5 years
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	503,113
Granted (in shares)	170,710
Vested (in shares)	(227,814)
Canceled (in shares)	(51,297)
Unvested restricted stock at end of period (in shares)	394,712	503,113
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 166.42
Granted (in dollars per share)	194.77
Vested (in dollars per share)	140.14
Canceled (in dollars per share)	177.72
Unvested restricted stock at end of period (in dollars per share)	$ 192.38	$ 166.42
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 3,000	$ 3,000	$ 1,000
Compensation cost expected to be recognized in future years	$ 5,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)	0.00%	0.00%	0.00%
Expected volatility (in hundredths)	28.00%	26.00%	30.00%
Risk-free interest rate (in hundredths)	1.00%	1.00%	1.00%
Expected life (in years)	3 years	3 years	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	54,960
Granted (in shares)	25,680	32,400	24,720
Vested (in shares)	0
Canceled (in shares)	(8,160)
Unvested restricted stock at end of period (in shares)	72,480	54,960
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 211.41
Granted (in dollars per share)	184.97	$ 208.08	$ 216.20
Vested (in dollars per share)	0.00
Canceled (in dollars per share)	211.18
Unvested restricted stock at end of period (in dollars per share)	$ 202.07	211.41
Minimum [Member] \| CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
Minimum [Member] \| CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
Minimum [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)	$ 36.48	36.48	36.48
Granted (in dollars per share)	182.75	193.69	201.04
Exercised (in dollars per share)	36.48	36.48	39.00
Canceled or expired (in dollars per share)	193.69	0.00	58.95
Outstanding at end of period, (in dollars per share)	36.48	36.48	36.48
Exercisable at end of period (in dollars per share)	$ 36.48	36.48	36.48
Minimum [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	1 year
Minimum [Member] \| Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	80.00%
Minimum [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	0.00%
Maximum [Member] \| CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
Maximum [Member] \| CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
Maximum [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)	$ 201.04	201.04	102.16
Granted (in dollars per share)	182.75	193.69	201.04
Exercised (in dollars per share)	201.04	201.04	102.16
Canceled or expired (in dollars per share)	201.04	0.00	201.04
Outstanding at end of period, (in dollars per share)	201.04	201.04	201.04
Exercisable at end of period (in dollars per share)	$ 201.04	$ 201.04	$ 102.16
Maximum [Member] \| Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	120.00%
Maximum [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	200.00%

EMPLOYEE BENEFIT PLANS (Details for Employee 401(k) Plan, Employee Pension Plan, Registered Retirement Savings Plan and Employee Stock Purchase Plan) - USD ($)		12 Months Ended
	Sep. 15, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee 401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%	100.00%	100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%	4.00%	4.00%
Company match to employee contributions		$ 9,000,000	$ 8,000,000	$ 6,000,000
Administrative expense		$ 0	$ 0	$ 0
Employee Pension Plan, London Office [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		6.00%	6.00%	6.00%
Company match to employee contributions		$ 380,000	$ 420,000	$ 390,000
Registered Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%
Company match to employee contributions		$ 10,000	$ 40,000
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of purchase price of company's common stock to the market price		90.00%
Increase in shares of common stock issued pursuant to stock plan (in shares)	100,000
Shares available, employee stock purchase plan		93,812	108,547
Shares of Company's common stock purchased during the period (in shares)		14,735	13,227