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Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2018	Apr. 20, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	0001057352
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		36,392,021

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues	$ 273,718	$ 226,553
Cost of revenues	62,477	51,346
Gross profit	211,241	175,207
Operating expenses:
Selling and marketing (excluding customer base amortization)	88,490	76,402
Software development	22,913	22,374
General and administrative	40,590	33,995
Customer base amortization	5,803	4,774
Total operating expenses	157,796	137,545
Income from operations	53,445	37,662
Interest and other income	2,987	429
Interest and other expense	(690)	(2,686)
Income before income taxes	55,742	35,405
Income tax expense	3,511	13,275
Net income	$ 52,231	$ 22,130
Net income per share-basic (in dollars per share)	$ 1.46	$ 0.69
Net income per share-diluted (in dollars per share)	$ 1.44	$ 0.68
Weighted average outstanding shares-basic (in shares)	35,893	32,276
Weighted average outstanding shares-diluted (in shares)	36,350	32,563

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 52,231	$ 22,130
Other comprehensive income, net of tax
Foreign currency translation adjustment	951	411
Total other comprehensive income	951	411
Total comprehensive income	$ 53,182	$ 22,541

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 931,429	$ 1,211,463
Accounts receivable, net of allowance for doubtful accounts of approximately $5,136 and $6,469 as of March 31, 2018 and December 31, 2017, respectively	68,914	60,900
Prepaid expenses and other current assets	31,661	15,572
Total current assets	1,032,004	1,287,935
Long-term investments	10,070	10,070
Deferred income taxes, net	4,579	5,431
Property and equipment, net	83,401	84,496
Goodwill	1,551,248	1,283,457
Intangible assets, net	313,861	182,892
Deferred commission costs	75,201	0
Deposits and other assets	11,755	6,179
Income tax receivable	12,981	12,981
Total assets	3,095,100	2,873,441
Current liabilities:
Accounts payable	7,845	9,262
Accrued wages and commissions	51,037	54,104
Accrued expenses	40,647	22,193
Deferred gain on the sale of building	2,523	2,523
Income taxes payable	9,257	8,166
Deferred rent	4,866	4,732
Deferred revenue	49,468	45,686
Total current liabilities	165,643	146,666
Deferred gain on the sale of building	15,561	16,192
Deferred rent	32,000	33,909
Deferred income taxes, net	65,456	12,070
Income taxes payable	15,476	13,354
Total liabilities	294,136	222,191
Stockholders' equity:
Total stockholders’ equity	2,800,964	2,651,250
Total liabilities and stockholders’ equity	$ 3,095,100	$ 2,873,441

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Allowance for doubtful accounts	$ 5,136	$ 6,469

CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Cumulative effect of adoption of new accounting standard	$ 54,464				$ 54,464
Balance at January 1, 2018	2,705,714	$ 361	$ 2,339,253	$ (9,020)	375,120
Balance (in shares) at Dec. 31, 2017		36,107,000
Balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Net income	52,231				52,231
Other comprehensive income	$ 951			951
Exercise of stock options (in shares)	110,791	111,000
Exercise of stock options	$ 9,328	$ 1	9,327
Restricted stock grants (in shares)		114,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(47,000)
Restricted stock grants surrendered	(15,392)	$ 0	(15,392)
Stock compensation expense, net of forfeitures	10,335		10,335
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,431	$ 0	1,431
Stock issued for acquisitions (in shares)		103,000
Stock issued for acquisitions	36,366	$ 1	36,365
Balance (in shares) at Mar. 31, 2018		36,392,000
Balance at Mar. 31, 2018	$ 2,800,964	$ 364	$ 2,381,318	$ (8,069)	$ 427,351

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net income	$ 52,231	$ 22,130
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	16,983	17,298
Amortization of deferred commissions costs	12,006	0
Amortization of debt issuance costs	219	772
Stock-based compensation expense	10,412	9,357
Deferred income tax expense, net	1,851	2,091
Bad debt expense	1,431	1,800
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(2,511)	(2,760)
Prepaid expenses and other current assets	(9,522)	(359)
Deferred commissions	(16,263)	0
Deposits and other assets	(3,412)	(117)
Accounts payable and other liabilities	4,288	11,588
Deferred revenue	5,272	2,757
Net cash provided by operating activities	72,985	64,557
Investing activities:
Purchases of property and equipment and other assets	(8,617)	(6,146)
Acquisitions, net of cash acquired	(340,074)	(13,673)
Net cash used in investing activities	(348,691)	(19,819)
Financing activities:
Payments of long-term debt	0	(35,000)
Repurchase of restricted stock to satisfy tax withholding obligations	(15,392)	(5,781)
Proceeds from exercise of stock options and employee stock purchase plan	10,616	1,234
Net cash used in financing activities	(4,776)	(39,547)
Effect of foreign currency exchange rates on cash and cash equivalents	448	58
Net (decrease) increase in cash and cash equivalents	(280,034)	5,249
Cash and cash equivalents at the beginning of period	1,211,463	567,223
Cash and cash equivalents at the end of period	931,429	572,472
Supplemental cash flow disclosures:
Interest paid	381	1,336
Income taxes paid	533	434
Supplemental non-cash investing and financing activities:
Stock issued in connection with acquisition - ForRent	$ 36,366	$ 0

ORGANIZATION	3 Months Ended
ORGANIZATION	Mar. 31, 2018
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 also 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

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

The condensed 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.

Interim Financial Statements

The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information. In the opinion of the Company’s management, the financial statements reflect all adjustments necessary to present fairly the Company’s financial position at March 31, 2018 and December 31, 2017, the results of its operations for the three months ended March 31, 2018 and 2017, its comprehensive income for the three months ended March 31, 2018 and 2017, its changes in stockholders equity for the three months ended March 31, 2018 and its cash flows for the three months ended March 31, 2018 and 2017. These adjustments are of a normal recurring nature.

Certain notes and other information have been condensed or omitted from the interim financial statements presented in this Quarterly Report on Form 10-Q. Therefore, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2017.

Use of Estimates

The preparation of financial statements in conformity with 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 loss 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

Subsequent to the Adoption of Accounting Standards Codification Revenue from Contracts with Customers (“ASC 606") on January 1, 2018

The Company derives revenues by (i) providing access to its proprietary database of commercial real estate information and (ii) providing an online marketplace for professional property management companies, property owners, brokers, and landlords typically through a fixed monthly fee for its subscription-based services. The Company's 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.

The Company analyzes contracts to determine the appropriate revenue recognition using the following steps: (i) identification of contracts with customers, (ii) identification of distinct performance obligations in the contract, (iii) determination of contract transaction price, (iv) allocation of contract transaction price to the performance obligations and (v) determination of revenue recognition based on timing of satisfaction of the performance obligation.

The Company recognizes revenues upon the satisfaction of its performance obligation (upon transfer of control of promised goods or services to our customers) in an amount that reflects the consideration to which it expects to be entitled to in exchange for those goods or services. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement.

The Company's contracts with customers often include promises to transfer multiple products and services. For these contracts, the Company accounts for individual performance obligations separately if they are distinct. Determining whether products and services are considered distinct performance obligations may require significant judgment. Judgment is required to determine the standalone selling price (“SSP”) for each distinct performance obligation. In instances where SSP is not directly observable, such as when the Company does not sell the product or service separately, the Company determines the SSP using information that may include market conditions and other observable inputs.

Deferred revenue results from amounts billed in advance to customers or cash received from customers in advance from the sale of subscription licenses and is recognized over the term of the license agreement.

Contract assets are generated when contractual billing schedules differ from revenue recognition timing. Contract assets represent a conditional right to consideration for satisfied performance obligations that becomes a billed receivable when the conditions are satisfied.

Certain sales commissions are considered incremental and recoverable costs of obtaining a contract with a customer. Sales commissions incurred for obtaining new contracts are deferred and then amortized as selling and marketing expenses on a straight-line basis over a period of benefit that the Company has determined to be three years. The three-year amortization period was determined based on several factors, including the nature of the technology and proprietary data underlying the services being purchased, customer contract renewal rates, and industry competition. See Note 3 for further discussion on the impact of the adoption.

For details about the Company’s revenue recognition policy prior to the adoption of ASC 606, refer to the Company’s Annual Report on Form 10-K for the year ended December 31, 2017.

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 transactions 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.

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 $23 million and $23 million for the three months ended March 31, 2018 and 2017, respectively.

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 condensed consolidated statements of operations. There were no material gains or losses from foreign currency exchange transactions for the three months ended March 31, 2018 and 2017.

Accumulated Other Comprehensive Loss

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



	March 31, 2018			December 31, 2017
Foreign currency translation adjustment	$	(7,339	)	$	(8,290	)
Accumulated net unrealized loss on investments, net of tax	(730		)	(730		)
Total accumulated other comprehensive loss	$	(8,069	)	$	(9,020	)

There were no amounts reclassified out of accumulated other comprehensive loss to the condensed consolidated statements of operations for the three months ended March 31, 2018 and 2017. The foreign currency translation adjustment includes intra-entity foreign currency transactions that are of a long-term investment nature.

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 Per Share

Net income per share is computed by dividing net income 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, unvested performance-based restricted stock and restricted stock units. Diluted net income 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.

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



	Three Months Ended March 31,
Numerator:	2018		2017
Numerator:
Net income	$	52,231	$	22,130
Denominator:
Denominator for basic net income per share — weighted-average outstanding shares	35,893		32,276
Effect of dilutive securities:
Stock options and restricted stock awards	457		287
Denominator for diluted net income per share — weighted-average outstanding shares	36,350		32,563

Net income per share — basic	$	1.46	$	0.69
Net income per share — diluted	$	1.44	$	0.68

Stock options to purchase approximately 83,000 and 253,000 shares that were outstanding for the three months ended March 31, 2018 and 2017, 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. Shares underlying restricted common stock awards that vest based on Company performance, market 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):



	Three Months Ended March 31,
	2018	2017
Performance-based restricted stock awards	84	85
Service-based restricted stock units	1	1
Total shares excluded from computation	85	86

Stock-Based Compensation

Equity instruments issued in exchange for services performed by officers, employees, and directors 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 condensed 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.

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



	Three Months Ended March 31,
	2018		2017
Cost of revenues	$	1,431	$	1,171
Selling and marketing	1,835		1,652
Software development	1,729		1,810
General and administrative	5,417		4,724
Total stock-based compensation	$	10,412	$	9,357

Options to purchase 110,791 and 8,290 shares were exercised during the three months ended March 31, 2018 and 2017, respectively.

The Company adopted the Management Stock Purchase Plan on December 7, 2017. This Plan is intended to provide selected key employees of the Company and its subsidiaries the opportunity to defer a portion of their bonus and commission compensation and to align management and shareholder interests through awards of Deferred Stock Units under this Plan and awards of matching Restricted Stock Units under Section 8 of the CoStar Group, Inc. 2016 Stock Incentive Plan or its successor plan(s) starting with the 2018 bonus year. The plan does not currently have an impact on the financial statements.

Debt Issuance Costs

Costs incurred in connection with the issuance of long-term debt are deferred and amortized as interest expense over the term of the related debt using the effective interest method for term debt and on a straight-line basis for revolving debt. These amounts are reflected in the consolidated balance sheets as long-term assets for costs related to revolving debt. Upon a refinancing or amendment, 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 appropriate method. The Company had capitalized debt issuance costs, net of amortization, of approximately $4 million and $4 million as of March 31, 2018 and December 31, 2017, respectively. The debt issuance costs are associated with the Company's previous credit agreements, and the current amended and restated 2017 Credit Agreement (the "2017 Credit Agreement"). See Note 9 for additional information regarding the revolving credit facility. The Company amortized debt issuance costs of approximately $219,000 and $772,000 for the three months ended March 31, 2018 and 2017, respectively.

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 and contingent liabilities. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, acquired trade names and building photography from a market participant's perspective, useful lives and discount rates. 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.

For a given acquisition, the Company may identify certain pre-acquisition contingencies as of the acquisition date and may extend its review and evaluation of these pre-acquisition contingencies throughout the measurement period in order to obtain sufficient information to assess whether the Company includes these contingencies as a part of the fair value estimates of assets acquired and liabilities assumed and, if so, to determine their estimated amounts.

If the Company cannot reasonably determine the fair value of a pre-acquisition contingency (non-income tax related) by the end of the measurement period, which is generally the case given the nature of such matters, the Company will recognize an asset or a liability for such pre-acquisition contingency if: (i) it is probable that an asset existed or a liability had been incurred at the acquisition date and (ii) the amount of the asset or liability can be reasonably estimated. Subsequent to the measurement period, changes in our estimates of such contingencies will affect earnings and could have a material effect on our results of operations and financial position.

In addition, uncertain tax positions and tax related valuation allowances assumed in connection with a business combination are initially estimated as of the acquisition date. The Company reevaluates these items based upon facts and circumstances that existed as of the acquisition date with any adjustments to our preliminary estimates being recorded to goodwill provided that the Company is within the measurement period. Subsequent to the measurement period, changes to these uncertain tax positions and tax related valuation allowances will affect our provision for income taxes in our consolidated statement of operations and comprehensive income and could have a material impact on our results of operations and financial position.

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

There have been no developments to the Recent Accounting Pronouncements discussion included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, including the expected dates of adoption and estimated effects on the Company’s condensed consolidated financial statements, except for the following:

In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard, Accounting Standards Update (“ASU") 2014-09, Revenue from Contracts with Customers (ASC 606) 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 is applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. This guidance is effective for annual reporting periods beginning after December 15, 2017.

On January 1, 2018, the Company adopted ASC 606, using the modified retrospective method. Results for reporting periods beginning subsequent to December 31, 2017 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with the Company’s historical accounting policies prior to adoption. In adopting the guidance, the Company applied the guidance to all contracts and used several available practical expedients including assessing contracts with similar terms and conditions on a “portfolio” basis and not including contracts with a duration of one year or less in the unsatisfied performance obligations disclosure.

The Company recorded a net cumulative effect to beginning retained earnings of $54 million. The Company adjusted the condensed consolidated financial statements from amounts previously reported due to the adoption of ASC 606. Select condensed consolidated balance sheet line items which were adjusted upon adoption were as follows (in thousands):



	As of December 31, 2017		ASC 606 Adjustments			As of January 1, 2018
Assets
Accounts receivable, net	$	60,900	$	(1,867	)	$	59,033
Prepaid expenses and other current assets	15,572		1,867			17,439
Deferred commissions costs	—		71,118			71,118
Liabilities
Deferred revenue	$	45,686	$	(1,716	)	$	43,970
Deferred income taxes, net	12,070		18,370			30,440
Retained earnings	320,656		54,464			375,120

The impact of the adoption of ASC 606 on the condensed financial statements during the three months ended March 31, 2018 was as follows (in thousands):



	As of March 31, 2018 without adoption of ASC 606		ASC 606 Adjustments			As reported as of March 31, 2018
Assets
Accounts receivable, net	$	74,550	$	(5,636	)	$	68,914
Prepaid expenses and other current assets	26,025		5,636			31,661
Deferred commissions costs	—		75,201			75,201
Liabilities
Deferred revenue	$	50,684	$	(1,216	)	$	49,468
Deferred income taxes, net	46,122		19,334			65,456
Retained earnings	370,268		57,083			427,351

If the Company had not adopted ASC 606, it would have recognized additional revenue of approximately $500 thousand and additional selling and marketing expense of $4 million for the three months ended March 31, 2018. The impact on net income and basic and diluted earnings per share for the period would have been a decrease of approximately $3 million and $0.08 per share, respectively.

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments, which is 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. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures.

In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business,

which is 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. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures.

In May 2017, the FASB issued ASU 2017-09, Compensation- Stock Compensation (Topic 718): Scope of Modification Accounting, which is designed to reduce the existing diversity and complexity in the accounting for changes to terms or conditions of a share-based payment award. This guidance clarifies that an entity will not apply modification accounting to a share-based payment award if all of the following are the same immediately before and after the change: (i) the fair value of the award, (ii) the vesting conditions of the award, and (iii) the classification of the award as an equity instrument or liability instrument. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures.

Recent Accounting Pronouncements Not Yet Adopted

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), 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 adoption is permitted. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures, and expects that the adoption of this standard may result in a material increase in assets and liabilities on its consolidated balance sheets.

REVENUE FROM CONTRACTS WITH CUSTOMERS

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue from Contract with Customers

REVENUE FROM CONTRACTS WITH CUSTOMERS

Disaggregated Revenue

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



	Three Months Ended March 31,
	2018		2017
Information and analytics
CoStar Suite	$	130,361	$	109,979
Information services	15,060		18,336
Online marketplaces
Multifamily	87,683		63,991
Commercial property and land	40,614		34,247
Total revenues	$	273,718	$	226,553

Revenue in the Company's North America segment comes from all types of services. However, predominately all revenue in the International segment comes from Information and analytics services.

Deferred Revenue

Changes in deferred revenue for the period were as follows (in thousands):



Balance at December 31, 2017	$	45,686
Cumulative effect of adoption of ASC 606	(1,716		)
Balance at January 1, 2018	43,970
Revenue recognized in the current period from the amounts in the beginning balance	(29,910		)
New deferrals, net of amounts recognized in the current period	35,173
Effects of foreign currency translation	235
Balance at March 31, 2018	$	49,468

Contract Assets

The Company had contract assets of $6 million and $2 million as of March 31, 2018 and January 1, 2018, which are generated when contractual billing schedules differ from revenue recognition timing. Contract assets represent a conditional right to consideration for satisfied performance obligations that becomes a billed receivable when the conditions are satisfied.

Commissions

The Company recognized $12 million of amortization of deferred commissions included in selling and marketing expense in the three months ended March 31, 2018. The Company determined that no deferred commissions were impaired in the three months ended March 31, 2018.

Commissions expense rollforward for the three months ended March 31, 2018 was as follows (in thousands):



Commissions expense prior to adoption	$	23,595
Commissions capitalized in the current period	(16,263		)
Amortization of deferred commissions costs	12,006
Total commissions expense	$	19,338

Unsatisfied Performance Obligations

Remaining contract consideration for which revenue has not been recognized due to unsatisfied performance obligations was approximately $89 million at March 31, 2018, which the Company expects to recognize over the next three years. This amount does not include contract consideration for contracts with a duration of one year or less.

ACQUISITION

3 Months Ended

Mar. 31, 2018

Business Combinations [Abstract]

ACQUISITION

On February 21, 2018 ("Acquisition Date"), the Company acquired all of the issued and outstanding capital stock of DE Holdings, Inc., including its ForRent division ("ForRent"), a wholly owned subsidiary of Dominion Enterprises ("Seller"), for a purchase price of approximately $376 million. The purchase price was comprised of approximately $340 million in cash and 103,280 in shares of Company common stock, valued at approximately $36 million, and is subject to a customary working capital adjustment. ForRent's primary service is digital advertising through a network of four multifamily websites. The acquisition is expected to yield increased revenue, significant cost synergies and an improved competitive position in the industry.

The Company applied the acquisition method to account for the ForRent transaction, which requires that 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	$	59
Accounts receivable	8,769
Indemnification asset	5,443
Goodwill	266,720
Intangible assets	141,300
Deferred tax liabilities	(34,032		)
Contingent sales tax liability	(6,260		)
State uncertain income tax position liability	(2,047		)
Other assets and liabilities	(3,453		)
Fair value of identifiable net assets acquired	$	376,499

The net assets of ForRent were recorded at their estimated fair value. In valuing acquired assets and liabilities, fair value estimates were based primarily on future expected cash flows, market rate assumptions for contractual obligations, and appropriate discount rates. The estimated fair values are preliminary and are subject to change within the measurement period.

The acquired customer base for the acquisition 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 three years. The acquired trade name has a weighted average estimated useful life of ten years. The acquired building photography had an estimated useful life of one year. 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 $267 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment. $8 million 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 ForRent 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 ForRent's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce.

The Company has assessed the (i) probability of a contingent sales tax liability and (ii) a state uncertain income tax position liability due to apportionment factors, and has recorded accruals of $6 million and $2 million, respectively. The Company could not determine the fair value for the pre-acquisition state sales tax liability and therefore estimated a liability in accordance with ASC 450, using a state-by-state assessment. The uncertain income tax position was determined in accordance with the provisions of ASC 740 and was recorded as part of purchase price allocation. The Seller has provided an indemnity for tax liabilities prior to the acquisition. Seller's indemnification for sales taxes in the state of Texas is limited to approximately $2 million. The total indemnification asset established as of the acquisition date is $5 million.

As part of the ForRent acquistion, the Company incurred $3 million of transaction costs. Additionally, the Company paid $12 million in cash in escrow for retention compensation for certain ForRent employees, provided they remain employed by the Company for a defined six month period following the acquisition or are earlier terminated without cause or resign for good reason. In the event some or all of those employees are not entitled to their retention bonus, those funds will be remitted to the Seller. The Company will expense the retention compensation as the services are performed in the post-combination period.

The unaudited pro forma financial information presented below summarizes the combined results of operations for the Company and ForRent as though the companies were combined as of January 1, 2017. The unaudited pro forma financial information for all periods presented include amortization charges from acquired intangible assets (certain of which are preliminary), retention compensation for stay pay, as referenced above, and the related tax effects, along with certain other accounting effects, but excludes the impacts of any expected operational synergies. The unaudited pro forma financial information as presented below is for informational purposes only and is not necessarily indicative of the results of operations that would have been achieved if the acquisition had taken place on January 1, 2017.

The unaudited pro forma financial information for the three months ended March 31, 2018 and 2017 combine the historical results of the Company for the three months ended March 31, 2018 and 2017 and the historical results of ForRent for the period January 1, 2018 through February 20, 2018 and three months ended March 31, 2017 and the effects of the pro forma adjustments listed above.

The unaudited pro forma financial information was as follows (in thousands, except per share data):



	March 31, 2018		March 31, 2017
Revenue	$	287,470	$	252,895
Net income	$	52,839	$	15,996
Net income per share - basic	$	1.47	$	0.49
Net income per share - diluted	$	1.45	$	0.49

Revenue and net loss attributable to ForRent from February 21, 2018 through March 31, 2018 were $8 million and $8 million, respectively. The net loss was primarily due to personnel costs, including retention compensation, and the amortization of intangible assets upon acquisition.

INVESTMENTS

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

INVESTMENTS

The Company determines the appropriate classification of debt 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.

Scheduled maturities of investments classified as available-for-sale as of March 31, 2018 are as follows (in thousands):



Maturity	Fair Value
Due:
April 1, 2018 — March 31, 2019	$	—
April 1, 2019 — March 31, 2023	—
April 1, 2023 — March 31, 2028	—
After March 31, 2028	10,070
Available-for-sale investments	$	10,070

The Company had no realized gains on its investments for each of the three months ended March 31, 2018 and 2017, respectively. The Company had no realized losses on its investments for each of the three months ended March 31, 2018 and 2017, respectively. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis.

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.

As of March 31, 2018, 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	$	—	$	(730	)	$	10,070
Available-for-sale investments	$	10,800	$	—	$	(730	)	$	10,070

As of December 31, 2017, 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	$	—	$	(730	)	$	10,070
Available-for-sale investments	$	10,800	$	—	$	(730	)	$	10,070

The unrealized losses on the Company’s investments as of March 31, 2018 and December 31, 2017 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 March 31, 2018 and December 31, 2017. See Note 6 for further discussion of the fair value of the Company’s financial assets.

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



	March 31, 2018					December 31, 2017
	Aggregate Fair Value		Gross Unrealized Losses			Aggregate Fair Value		Gross Unrealized Losses
Auction rate securities	$	10,070	$	(730	)	$	10,070	$	(730	)
Investments in an unrealized loss position	$	10,070	$	(730	)	$	10,070	$	(730	)

The Company did not have any investments in an unrealized loss position for less than twelve months as of March 31, 2018 and December 31, 2017.

FAIR VALUE

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 (in thousands):



	Level 1		Level 2		Level 3		Total
Assets:
Money market	$	584,244	$	—	$	—	$	584,244
Auction rate securities	—		—		10,070		10,070
Total assets measured at fair value	$	584,244	$	—	$	10,070	$	594,314

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, 2017 (in thousands):



	Level 1		Level 2		Level 3		Total
Assets:
Money market	$	586,084	$	—	$	—	$	586,084
Auction rate securities	—		—		10,070		10,070
Total assets measured at fair value	$	586,084	$	—	$	10,070	$	596,154

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.

The following tables summarize changes in fair value of the Company’s Level 3 assets for the three months ended March 31, 2018 and 2017 (in thousands):



	Three Months Ended March 31,
	2018		2017
Balance at beginning of period	$	10,070	$	9,952
Decrease in unrealized loss included in accumulated other comprehensive loss	—		—
Settlements	—		—
Balance at end of period	$	10,070	$	9,952

The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2016 to March 31, 2018 (in thousands):



	Auction Rate Securities
Balance at December 31, 2016	$	9,952
Decrease in unrealized loss included in accumulated other comprehensive loss	118
Balance at December 31, 2017	10,070
Decrease in unrealized loss included in accumulated other comprehensive loss	—
Settlements	—
Balance at March 31, 2018	$	10,070

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

As of March 31, 2018, 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 AA to 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 condensed consolidated balance sheet as of March 31, 2018. See Note 5 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 March 31, 2018. 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 March 31, 2018 and December 31, 2017 was approximately 6%. 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 March 31, 2018, the Company determined there was no decline in the fair value of its ARS investments. In addition, while a majority of the ARS are currently rated AA to AAA, if the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, then 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

GOODWILL

3 Months Ended

Mar. 31, 2018

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, 2016	$	1,227,777	$	27,089	$	1,254,866
Acquisitions	25,717		—		25,717
Effect of foreign currency translation	—		2,874		2,874
Goodwill, December 31, 2017	1,253,494		29,963		1,283,457
Acquisition	266,720		—		266,720
Effect of foreign currency translation	—		1,071		1,071
Goodwill, March 31, 2018	$	1,520,214	$	31,034	$	1,551,248

The Company recorded goodwill of approximately $8 million in connection with the January 31, 2017 acquisition of Koa Lei, Inc. (doing business as Westside Rentals and now known as Westside Rentals, LLC), an online marketplace specializing in Southern California real estate rentals, and its affiliated entity Westside Credit Services, LLC, a provider of credit checks and tenant screening for landlords in the Southern California real estate rental market. The Company recorded goodwill of approximately $15 million in connection with the May 10, 2017 acquisition of certain assets and assumption of certain liabilities from Datasphere Technologies, Inc., in each case, related to the LandWatch.com® business (collectively referred to as “LandWatch”), a leading listing site dedicated to land and rural properties. The Company recorded goodwill of approximately $2 million in connection with the July 18, 2017 acquisition of The Screening Pros, LLC, an online apartment leasing platform that includes tenant screening services, rental applications and payments processing and lease renewals. The purchase accounting for the acquisitions of LandWatch® and The Screening Pros^TM is preliminary, subject to the completion of the accounting for certain tax related items and working capital adjustments. The Company recorded goodwill of approximately $267 million in connection with the February 21, 2018 acquisition of ForRent, a digital advertising service provided through a network of four multifamily websites. The purchase accounting for the acquisition of ForRent is preliminary, subject to the completion of the accounting for certain tax related items and working capital adjustments.

INTANGIBLE ASSETS

3 Months Ended

Mar. 31, 2018

Intangible Assets, Net (Excluding Goodwill) [Abstract]

INTANGIBLE ASSETS

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



	March 31, 2018			December 31, 2017			Weighted- Average Amortization Period (in years)
Capitalized product development cost	$	2,173		$	2,275		4
Accumulated amortization	(2,165		)	(2,262		)
Capitalized product development cost, net	8			13

Building photography	8,935			18,739			2
Accumulated amortization	(8,339		)	(18,212		)
Building photography, net	596			527

Acquired database technology	93,561			83,469			4
Accumulated amortization	(80,405		)	(79,188		)
Acquired database technology, net	13,156			4,281

Acquired customer base	336,376			225,879			10
Accumulated amortization	(175,426		)	(169,157		)
Acquired customer base, net	160,950			56,722

Acquired trade names and other intangible assets	188,856			167,718			13
Accumulated amortization	(49,705		)	(46,369		)
Acquired trade names and other intangible assets, net	139,151			121,349

Intangible assets, net	$	313,861		$	182,892

LONG-TERM DEBT	3 Months Ended
LONG-TERM DEBT	Mar. 31, 2018
Debt Disclosure [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT On October 19, 2017, the Company entered into an amended and restated credit agreement (the ‘‘2017 Credit Agreement’’), which amended and restated in its entirety the existing credit agreement dated April 1, 2014 (the "2014 Credit Agreement"). The 2017 Credit Agreement provides for a $750 million revolving credit facility with a term of five years from a syndicate of financial institutions as lenders and issuing banks. The 2017 facility may be used for working capital and other general corporate purposes of the Company and its subsidiaries. In connection with the transaction, the Company incurred $4 million of issuance costs. Those costs along with the $5 million of unamortized costs related to the prior agreement were allocated between the extinguishment of the 2014 Credit Agreement and the 2017 Credit Agreement. This allocation resulted in the Company recognizing a loss of $4 million on the extinguishment with the remaining $4 million being deferred and amortized on a straight-line basis as interest expense over the term of the 2017 Credit Agreement. Up to $20 million of the revolving credit facility is available for the issuance of letters of credit. The Company has an irrevocable standby letter of credit outstanding totaling $0.2 million as of March 31, 2018 and December 31, 2017, respectively, which was required to secure its San Francisco office lease. The letter of credit was established in 2014 and automatically renews through January 31, 2025. The loans under the 2017 Credit Agreement bear interest during any interest period selected by the Company, at either (i) 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 1.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2017 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 ½ of 1% and (z) LIBOR for a one-month interest period plus 1.00%, plus an initial spread of 0.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurs under the 2017 Credit Agreement, the interest rate on overdue amounts will increase by 2.00% per annum. The obligations under the 2017 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 its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee agreements entered into on the closing date of the 2017 Credit Agreement. The 2017 Credit Agreement requires the Company to maintain (i) a First Lien Secured Leverage Ratio not exceeding 3.50 to 1.00 and (ii) after the incurrence of additional indebtedness under certain specified exceptions in the 2017 Credit Agreement, a Total Leverage Ratio (as defined in the 2017 Credit Agreement) not exceeding 4.50 to 1.00. The 2017 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 2017 Credit Agreement as of March 31, 2018. The Company had no outstanding long-term debt at March 31, 2018 and December 31, 2017. Total interest expense for the term loan and revolving credit facilities was approximately $1 million for three months ended March 31, 2018, which was comprised of amortization of debt issuance costs and commitment fees, and $3 million for the three months ended March 31, 2017. The Company had $4 million and $4 million of deferred debt issuance costs included in deposits and other assets at March 31, 2018 and December 31, 2017, respectively.

INCOME TAXES	3 Months Ended
INCOME TAXES	Mar. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The income tax provision for the three months ended March 31, 2018 and 2017 reflects an effective tax rate of approximately 6% and 37%, respectively. The decrease in the effective tax rate for the three months ended March 31, 2018 compared to the three months ended March 31, 2017 was primarily due to the Tax Cuts and Jobs Act (the “Tax Act”), which reduced the U.S. federal corporate income tax rate from 35% to 21% effective January 1, 2018, and excess tax benefits on share-based payments. The Company’s accounting for the tax effects of the Tax Act is incomplete. The Securities and Exchange Commission staff issued Staff Accounting Bulletin 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting under Accounting Standards Codification Topic 740, (“ASC 740”). In accordance with SAB 118, a company must reflect the income tax effects of those aspects of the Tax Act for which the accounting under ASC 740 is complete. To the extent that a company’s accounting for certain income tax effects of the Tax Act is incomplete but it is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply ASC 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the Tax Act. As noted in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, at year-end, the Company was able to reasonably estimate certain tax effects of the Tax Act and, therefore, recorded provisional adjustments. The Company has not made any additional measurement-period adjustments related to the tax effects of the Tax Act during the three months ended March 31, 2018. The Company is continuing to gather additional information to complete its accounting for the tax effects of the Tax Act and expects to complete such accounting within the prescribed measurement period of up to one year from the enactment of the Tax Act.

COMMITMENTS AND CONTINGENCIES	3 Months Ended
COMMITMENTS AND CONTINGENCIES	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES The Company leases office facilities and office equipment under various non-cancelable operating leases. The leases contain various renewal options. Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. 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 condensed consolidated financial statements for unfavorable results, if any. Legal defense costs are expensed as incurred.

SEGMENT REPORTING

3 Months Ended

Mar. 31, 2018

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. 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 before interest, loss on debt extinguishment, 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):



	Three Months Ended March 31,
	2018			2017
Revenues
North America	$	264,795		$	219,341
International
External customers	8,923			7,212
Intersegment revenues	13			11
Total International revenues	8,936			7,223
Intersegment eliminations	(13		)	(11		)
Total revenues	$	273,718		$	226,553

EBITDA
North America	$	71,055		$	54,433
International	(627		)	527
Total EBITDA	$	70,428		$	54,960

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



	Three Months Ended March 31,
	2018			2017
Net income	$	52,231		$	22,130
Amortization of acquired intangible assets in cost of revenues	4,608			6,119
Amortization of acquired intangible assets in operating expenses	5,803			4,774
Depreciation and other amortization	6,572			6,405
Interest and other income	(2,987		)	(429		)
Interest and other expense	690			2,686
Income tax expense	3,511			13,275
EBITDA	$	70,428		$	54,960

Intersegment revenues recorded 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 for CoStar Portfolio Strategy.

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



	March 31, 2018		December 31, 2017
Property and equipment, net
North America	$	78,722	$	79,736
International	4,679		4,760
Total property and equipment, net	$	83,401	$	84,496

Goodwill
North America	$	1,520,214	$	1,253,494
International	31,034		29,963
Total goodwill	$	1,551,248	$	1,283,457

Assets
North America	$	3,037,575	$	2,816,156
International	57,525		57,285
Total assets	$	3,095,100	$	2,873,441

Liabilities
North America	$	274,776	$	201,831
International	19,360		20,360
Total liabilities	$	294,136	$	222,191

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Revenue Recognition

Subsequent to the Adoption of Accounting Standards Codification Revenue from Contracts with Customers (“ASC 606") on January 1, 2018

Cost of Revenues

Advertising Costs

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

Foreign Currency Translation

Income Taxes

Net Income Per Share

Shares underlying restricted common stock awards that vest based on Company performance, market 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.

Net Income Per Share

Stock-Based Compensation

Debt Issuance Costs

Business Combinations

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements



	As of December 31, 2017		ASC 606 Adjustments			As of January 1, 2018
Assets
Accounts receivable, net	$	60,900	$	(1,867	)	$	59,033
Prepaid expenses and other current assets	15,572		1,867			17,439
Deferred commissions costs	—		71,118			71,118
Liabilities
Deferred revenue	$	45,686	$	(1,716	)	$	43,970
Deferred income taxes, net	12,070		18,370			30,440
Retained earnings	320,656		54,464			375,120

The impact of the adoption of ASC 606 on the condensed financial statements during the three months ended March 31, 2018 was as follows (in thousands):



	As of March 31, 2018 without adoption of ASC 606		ASC 606 Adjustments			As reported as of March 31, 2018
Assets
Accounts receivable, net	$	74,550	$	(5,636	)	$	68,914
Prepaid expenses and other current assets	26,025		5,636			31,661
Deferred commissions costs	—		75,201			75,201
Liabilities
Deferred revenue	$	50,684	$	(1,216	)	$	49,468
Deferred income taxes, net	46,122		19,334			65,456
Retained earnings	370,268		57,083			427,351

In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business,

Recent Accounting Pronouncements Not Yet Adopted

Commitments and Contingencies

Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. 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.

Investment

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Schedule of accumulated other comprehensive loss

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



	March 31, 2018			December 31, 2017
Foreign currency translation adjustment	$	(7,339	)	$	(8,290	)
Accumulated net unrealized loss on investments, net of tax	(730		)	(730		)
Total accumulated other comprehensive loss	$	(8,069	)	$	(9,020	)

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

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



	Three Months Ended March 31,
Numerator:	2018		2017
Numerator:
Net income	$	52,231	$	22,130
Denominator:
Denominator for basic net income per share — weighted-average outstanding shares	35,893		32,276
Effect of dilutive securities:
Stock options and restricted stock awards	457		287
Denominator for diluted net income per share — weighted-average outstanding shares	36,350		32,563

Net income per share — basic	$	1.46	$	0.69
Net income per share — diluted	$	1.44	$	0.68

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):



	Three Months Ended March 31,
	2018	2017
Performance-based restricted stock awards	84	85
Service-based restricted stock units	1	1
Total shares excluded from computation	85	86

Stock-based compensation expense for stock options and restricted stock



	Three Months Ended March 31,
	2018		2017
Cost of revenues	$	1,431	$	1,171
Selling and marketing	1,835		1,652
Software development	1,729		1,810
General and administrative	5,417		4,724
Total stock-based compensation	$	10,412	$	9,357

Schedule of New Accounting Pronouncements and Changes in Accounting Principles

Select condensed consolidated balance sheet line items which were adjusted upon adoption were as follows (in thousands):



	As of December 31, 2017		ASC 606 Adjustments			As of January 1, 2018
Assets
Accounts receivable, net	$	60,900	$	(1,867	)	$	59,033
Prepaid expenses and other current assets	15,572		1,867			17,439
Deferred commissions costs	—		71,118			71,118
Liabilities
Deferred revenue	$	45,686	$	(1,716	)	$	43,970
Deferred income taxes, net	12,070		18,370			30,440
Retained earnings	320,656		54,464			375,120

The impact of the adoption of ASC 606 on the condensed financial statements during the three months ended March 31, 2018 was as follows (in thousands):



	As of March 31, 2018 without adoption of ASC 606		ASC 606 Adjustments			As reported as of March 31, 2018
Assets
Accounts receivable, net	$	74,550	$	(5,636	)	$	68,914
Prepaid expenses and other current assets	26,025		5,636			31,661
Deferred commissions costs	—		75,201			75,201
Liabilities
Deferred revenue	$	50,684	$	(1,216	)	$	49,468
Deferred income taxes, net	46,122		19,334			65,456
Retained earnings	370,268		57,083			427,351

REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue



	Three Months Ended March 31,
	2018		2017
Information and analytics
CoStar Suite	$	130,361	$	109,979
Information services	15,060		18,336
Online marketplaces
Multifamily	87,683		63,991
Commercial property and land	40,614		34,247
Total revenues	$	273,718	$	226,553

Contract with Customer, Asset and Liability

Changes in deferred revenue for the period were as follows (in thousands):



Balance at December 31, 2017	$	45,686
Cumulative effect of adoption of ASC 606	(1,716		)
Balance at January 1, 2018	43,970
Revenue recognized in the current period from the amounts in the beginning balance	(29,910		)
New deferrals, net of amounts recognized in the current period	35,173
Effects of foreign currency translation	235
Balance at March 31, 2018	$	49,468

Schedule of Commissions Expense

Commissions expense rollforward for the three months ended March 31, 2018 was as follows (in thousands):



Commissions expense prior to adoption	$	23,595
Commissions capitalized in the current period	(16,263		)
Amortization of deferred commissions costs	12,006
Total commissions expense	$	19,338

ACQUISITION (Tables)

3 Months Ended

Mar. 31, 2018

Business Combinations [Abstract]

Schedule of Recognized Identified 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	$	59
Accounts receivable	8,769
Indemnification asset	5,443
Goodwill	266,720
Intangible assets	141,300
Deferred tax liabilities	(34,032		)
Contingent sales tax liability	(6,260		)
State uncertain income tax position liability	(2,047		)
Other assets and liabilities	(3,453		)
Fair value of identifiable net assets acquired	$	376,499

Business Acquisition, Pro Forma Information

The unaudited pro forma financial information was as follows (in thousands, except per share data):



	March 31, 2018		March 31, 2017
Revenue	$	287,470	$	252,895
Net income	$	52,839	$	15,996
Net income per share - basic	$	1.47	$	0.49
Net income per share - diluted	$	1.45	$	0.49

INVESTMENTS (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 are as follows (in thousands):



Maturity	Fair Value
Due:
April 1, 2018 — March 31, 2019	$	—
April 1, 2019 — March 31, 2023	—
April 1, 2023 — March 31, 2028	—
After March 31, 2028	10,070
Available-for-sale investments	$	10,070

Schedule of available for sale securities reconciliation

As of March 31, 2018, 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	$	—	$	(730	)	$	10,070
Available-for-sale investments	$	10,800	$	—	$	(730	)	$	10,070

As of December 31, 2017, 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	$	—	$	(730	)	$	10,070
Available-for-sale investments	$	10,800	$	—	$	(730	)	$	10,070

Schedule of unrealized loss on investments for twelve months or longer

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



	March 31, 2018					December 31, 2017
	Aggregate Fair Value		Gross Unrealized Losses			Aggregate Fair Value		Gross Unrealized Losses
Auction rate securities	$	10,070	$	(730	)	$	10,070	$	(730	)
Investments in an unrealized loss position	$	10,070	$	(730	)	$	10,070	$	(730	)

FAIR VALUE (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Summary of fair value hierarchy for its financial assets and liabilities measured at fair value on a recurring basis



	Level 1		Level 2		Level 3		Total
Assets:
Money market	$	584,244	$	—	$	—	$	584,244
Auction rate securities	—		—		10,070		10,070
Total assets measured at fair value	$	584,244	$	—	$	10,070	$	594,314



	Level 1		Level 2		Level 3		Total
Assets:
Money market	$	586,084	$	—	$	—	$	586,084
Auction rate securities	—		—		10,070		10,070
Total assets measured at fair value	$	586,084	$	—	$	10,070	$	596,154

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

The following tables summarize changes in fair value of the Company’s Level 3 assets for the three months ended March 31, 2018 and 2017 (in thousands):



	Three Months Ended March 31,
	2018		2017
Balance at beginning of period	$	10,070	$	9,952
Decrease in unrealized loss included in accumulated other comprehensive loss	—		—
Settlements	—		—
Balance at end of period	$	10,070	$	9,952

The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2016 to March 31, 2018 (in thousands):



	Auction Rate Securities
Balance at December 31, 2016	$	9,952
Decrease in unrealized loss included in accumulated other comprehensive loss	118
Balance at December 31, 2017	10,070
Decrease in unrealized loss included in accumulated other comprehensive loss	—
Settlements	—
Balance at March 31, 2018	$	10,070

GOODWILL (Tables)

3 Months Ended

Mar. 31, 2018

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, 2016	$	1,227,777	$	27,089	$	1,254,866
Acquisitions	25,717		—		25,717
Effect of foreign currency translation	—		2,874		2,874
Goodwill, December 31, 2017	1,253,494		29,963		1,283,457
Acquisition	266,720		—		266,720
Effect of foreign currency translation	—		1,071		1,071
Goodwill, March 31, 2018	$	1,520,214	$	31,034	$	1,551,248

INTANGIBLE ASSETS (Tables)

3 Months Ended

Mar. 31, 2018

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):



	March 31, 2018			December 31, 2017			Weighted- Average Amortization Period (in years)
Capitalized product development cost	$	2,173		$	2,275		4
Accumulated amortization	(2,165		)	(2,262		)
Capitalized product development cost, net	8			13

Building photography	8,935			18,739			2
Accumulated amortization	(8,339		)	(18,212		)
Building photography, net	596			527

Acquired database technology	93,561			83,469			4
Accumulated amortization	(80,405		)	(79,188		)
Acquired database technology, net	13,156			4,281

Acquired customer base	336,376			225,879			10
Accumulated amortization	(175,426		)	(169,157		)
Acquired customer base, net	160,950			56,722

Acquired trade names and other intangible assets	188,856			167,718			13
Accumulated amortization	(49,705		)	(46,369		)
Acquired trade names and other intangible assets, net	139,151			121,349

Intangible assets, net	$	313,861		$	182,892

SEGMENT REPORTING (Tables)

3 Months Ended

Mar. 31, 2018

Segment Reporting [Abstract]

Summarized information by operating segment

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



	Three Months Ended March 31,
	2018			2017
Revenues
North America	$	264,795		$	219,341
International
External customers	8,923			7,212
Intersegment revenues	13			11
Total International revenues	8,936			7,223
Intersegment eliminations	(13		)	(11		)
Total revenues	$	273,718		$	226,553

EBITDA
North America	$	71,055		$	54,433
International	(627		)	527
Total EBITDA	$	70,428		$	54,960

Reconciliation of net income to EBITDA

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



	Three Months Ended March 31,
	2018			2017
Net income	$	52,231		$	22,130
Amortization of acquired intangible assets in cost of revenues	4,608			6,119
Amortization of acquired intangible assets in operating expenses	5,803			4,774
Depreciation and other amortization	6,572			6,405
Interest and other income	(2,987		)	(429		)
Interest and other expense	690			2,686
Income tax expense	3,511			13,275
EBITDA	$	70,428		$	54,960

Summarized information by operating segment, assets and liabilities

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



	March 31, 2018		December 31, 2017
Property and equipment, net
North America	$	78,722	$	79,736
International	4,679		4,760
Total property and equipment, net	$	83,401	$	84,496

Goodwill
North America	$	1,520,214	$	1,253,494
International	31,034		29,963
Total goodwill	$	1,551,248	$	1,283,457

Assets
North America	$	3,037,575	$	2,816,156
International	57,525		57,285
Total assets	$	3,095,100	$	2,873,441

Liabilities
North America	$	274,776	$	201,831
International	19,360		20,360
Total liabilities	$	294,136	$	222,191

ORGANIZATION (Details)	3 Months Ended
ORGANIZATION (Details)	Mar. 31, 2018 operating_segments
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of business segments	2
Term of subscription-based license agreements (in years)	1 year

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, REVENUE RECOGNITION AND ADVERTISING COSTS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)	1 year
Advertising expense	$ 23	$ 23

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, FOREIGN CURRENCY AND ACCUMULATED OTHER COMPREHENSIVE LOSS, NET OF TAX (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Material gains or losses from foreign currency transactions	$ 0
Accumulated Other Comprehensive Loss Net of Tax [Abstract]
Foreign currency translation adjustment	(7,339,000)	$ (8,290,000)
Accumulated net unrealized loss on investments, net of tax	(730,000)	(730,000)
Total accumulated other comprehensive loss	(8,069,000)	$ (9,020,000)
Reclassification out of accumulated other comprehensive loss	$ 0

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, NET INCOME PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator: [Abstract]
Net income	$ 52,231	$ 22,130
Denominator: [Abstract]
Denominator for basic net income per share - weighted-average outstanding shares (in shares)	35,893	32,276
Effect of dilutive securities: [Abstract]
Stock options and restricted stock	457	287
Denominator for diluted net income per share - weighted-average outstanding shares (in shares)	36,350	32,563
Net income per share - basic (in dollars per share)	$ 1.46	$ 0.69
Net income per share - diluted (in dollars per share)	$ 1.44	$ 0.68
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	85	86
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	83	253
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	84	85
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	1	1

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, STOCK BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 10,412	$ 9,357
Exercise of stock options (in shares)	110,791	8,290
Cost of Revenues [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 1,431	$ 1,171
Selling and Marketing [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	1,835	1,652
Software Development [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	1,729	1,810
General and Administrative [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 5,417	$ 4,724

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, DEBT ISSUANCE COSTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net [Abstract]
Capitalized debt issuance costs	$ 3,990		$ 4,208
Amortization of debt issuance costs	$ 219	$ 772

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , RECENT ACCOUNTING PRONOUCEMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard			$ 54,464
Revenues	$ 273,718	$ 226,553
Commissions	19,338
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard			$ 54,464
Retained Earnings [Member] \| Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard				$ (54,000)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	1,000
Commissions	23,595
Calculated under Revenue Guidance in Effect before Topic 606 [Member] \| Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Commissions	$ 4,000

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , EFFECT OF NEW ACCOUNTING STANDARD (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 68,914	$ 59,033	$ 60,900
Prepaid expenses and other current assets	31,661	17,439	15,572
Deferred commission costs	75,201	71,118	0
Deferred revenue	49,468	43,970	45,686
Deferred income taxes, net	65,456	30,440	12,070
Retained earnings	427,351	375,120
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	74,550		60,900
Prepaid expenses and other current assets	26,025		15,572
Deferred commission costs	0		0
Deferred revenue	50,684		45,686
Deferred income taxes, net	46,122		12,070
Retained earnings	370,268		$ 320,656
Accounting Standards Update 2014-09 [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	(5,636)	(1,867)
Prepaid expenses and other current assets	5,636	1,867
Deferred commission costs	75,201	71,118
Deferred revenue	(1,216)	(1,716)
Deferred income taxes, net	19,334	18,370
Retained earnings	$ 57,083	$ 54,464

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , IMPACT OF ADOPTION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 68,914	$ 59,033	$ 60,900
Prepaid expenses and other current assets	31,661	17,439	15,572
Deferred commission costs	75,201	71,118	0
Deferred revenue	49,468	43,970	45,686
Deferred income taxes, net	65,456	30,440	12,070
Retained earnings	427,351	375,120
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	74,550		60,900
Prepaid expenses and other current assets	26,025		15,572
Deferred commission costs	0		0
Deferred revenue	50,684		45,686
Deferred income taxes, net	46,122		12,070
Retained earnings	370,268		$ 320,656
Accounting Standards Update 2014-09 [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	(5,636)	(1,867)
Prepaid expenses and other current assets	5,636	1,867
Deferred commission costs	75,201	71,118
Deferred revenue	(1,216)	(1,716)
Deferred income taxes, net	19,334	18,370
Retained earnings	57,083	$ 54,464
Net income (loss)	$ (3,000)
Earnings per share, basic and diluted (usd per share)	$ (0.08)

REVENUE FROM CONTRACTS WITH CUSTOMERS , DISAGGREGATED REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 273,718	$ 226,553
Information And Analytics [Member] \| CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues	130,361	109,979
Information And Analytics [Member] \| Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues	15,060	18,336
Online Marketplaces [Member] \| Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues	87,683	63,991
Online Marketplaces [Member] \| Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues	$ 40,614	$ 34,247

REVENUE FROM CONTRACTS WITH CUSTOMERS , CONTRACT ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018
Change in Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 45,686	$ 45,686
Cumulative effect of adoption of ASC 606	(1,716)
Revenue recognized in the current period from the amounts in the beginning balance		(29,910)
New deferrals, net of amounts recognized in the current period		35,173
Effects of foreign currency translation		235
Ending balance	$ 43,970	$ 49,468

REVENUE FROM CONTRACTS WITH CUSTOMERS , COMMISSIONS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization of deferred commissions costs	$ 12,006	$ 0
Commissions	19,338
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization of deferred commissions costs	12,006
Commissions	23,595
Restatement Adjustment [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Commissions	$ (16,263)

REVENUE FROM CONTRACTS WITH CUSTOMERS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018
Revenue from Contract with Customer [Abstract]
Contract with customer, asset, gross	$ 6	$ 2
Revenue, remaining performance obligation	$ 89
Revenue, remaining performance obligation, expected timing of satisfaction, period	3 years

ACQUISITION (Details) - USD ($)		1 Months Ended	3 Months Ended
ACQUISITION (Details) - USD ($)	Feb. 21, 2018	Mar. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2015
Business Acquisition [Line Items]
Goodwill		$ 1,551,248,000	$ 1,551,248,000	$ 1,283,457,000	$ 1,254,866,000
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 376,000,000
Cash payment	$ 340,000,000
Investment owned (shares)	103,280
Purchase price, shares issued	$ 36,000,000
Goodwill	266,720,000					$ 267,000,000
Goodwill, expected tax deductible amount		8,000,000	$ 8,000,000
Indemnification asset	6,260,000
State uncertain income tax position liability	2,047,000
Indemnification asset	5,443,000
Transaction costs	3,000,000
Revenue since acquisition		8,000,000
Earnings since acquisition		$ (8,000,000)
Customer Relationships [Member] \| ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			10 years
Database Rights [Member] \| ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			3 years
Acquired trade names and other intangible assets [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			13 years
Acquired trade names and other intangible assets [Member] \| ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			10 years
Acquired building photography [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			2 years
Acquired building photography [Member] \| ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period			1 year
Maximum [Member] \| ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
State uncertain income tax position liability	2,000,000
Employee retention bonus	$ 11,627,000

ACQUISITION , SCHEDULE OF IDENTIFIED ASSETS AND LIABILITIES ASSUMED (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Feb. 21, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,551,248		$ 1,283,457	$ 1,254,866
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 59
Accounts receivable		8,769
Indemnification asset		5,443
Goodwill		266,720			$ 267,000
Intangible assets		141,300
Deferred tax liabilities		(34,032)
Contingent sales tax liability		(6,260)
State uncertain income tax position liability		2,047
Other assets and liabilities		(3,453)
Fair value of identifiable net assets acquired		$ 376,499

ACQUISITION , SCHEDULE OF PRO FORMA INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Combinations [Abstract]
Revenue	$ 287,470	$ 252,895
Net income	$ 52,839	$ 15,996
Net income per share - basic (usd per share)	$ 1.47	$ 0.49
Net income per share - diluted (usd per share)	$ 1.45	$ 0.49

INVESTMENTS, SCHEDULED MATURITIES AND REALIZED GAINS AND LOSSES (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Debt Maturities Fair Value [Abstract]
April 1, 2018 — March 31, 2019	$ 0
April 1, 2019 — March 31, 2023	0
April 1, 2023 — March 31, 2028	0
After March 31, 2028	10,070,000
Available-for-sale investments	10,070,000		$ 10,070,000
Available-for-sale securities, gross realized gains	0	$ 0
Available-for-sale securities, gross realized losses	$ 0	$ 0

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	$ 10,800,000	$ 10,800,000
Gross unrealized gains	0	0
Gross unrealized losses	(730,000)	(730,000)
Fair value	10,070,000	10,070,000
Available-For-Sale Securities, Unrealized Loss Positions [Abstract]
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	10,070,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	(730,000)	(730,000)
Continuous unrealized loss position, less than 12 months, accumulated loss	0	0
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	10,800,000	10,800,000
Gross unrealized gains	0	0
Gross unrealized losses	(730,000)	(730,000)
Fair value	10,070,000	10,070,000
Available-For-Sale Securities, Unrealized Loss Positions [Abstract]
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	10,070,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	$ (730,000)	$ (730,000)

FAIR VALUE (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
FAIR VALUE (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	$ 10,070	$ 9,952	$ 9,952
Change in unrealized gain (loss) included in accumulated other comprehensive loss	0	0
Settlements	0	0
Ending balance	10,070	9,952	10,070
Temporary impairment of the auction rates security investments, net of unrealized gain	(730)		(730)
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	10,070	$ 9,952	9,952
Change in unrealized gain (loss) included in accumulated other comprehensive loss	0		118
Settlements	0
Ending balance	$ 10,070		$ 10,070
Auction rate securities variable rate debt instruments interest rate reset period	28 days
The minimum contractual maturities on underlying securities involved in auction rate securities	20 years
Par value of company held auction rate securities	$ 11,000
Discount rate	6.00%		5.00%
Temporary impairment of the auction rates security investments, net of unrealized gain	$ 0
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets measured at fair value	594,314		$ 596,154
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	584,244		586,084
Fair Value, Measurements, Recurring [Member] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	10,070		10,070
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	584,244		586,084
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Assets:
Total assets measured at fair value	584,244		586,084
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] \| 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	10,070		10,070
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] \| Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	$ 10,070		$ 10,070

GOODWILL (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
GOODWILL (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,283,457	$ 1,254,866
Goodwill acquired	266,720	25,717
Effect of foreign currency translation	1,071	2,874
Goodwill, ending balance	1,551,248	1,283,457
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	1,253,494	1,227,777
Goodwill acquired	266,720	25,717
Effect of foreign currency translation	0	0
Goodwill, ending balance	1,520,214	1,253,494
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	29,963	27,089
Goodwill acquired	0	0
Effect of foreign currency translation	1,071	2,874
Goodwill, ending balance	$ 31,034	$ 29,963

GOODWILL (Narrative) (Details) - USD ($) $ in Thousands					3 Months Ended	12 Months Ended
GOODWILL (Narrative) (Details) - USD ($) $ in Thousands	Feb. 21, 2018	Jul. 18, 2017	May 10, 2017	Jan. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill acquired					$ 266,720	$ 25,717
Westside Rentals [Member]
Business Acquisition [Line Items]
Goodwill acquired				$ 8,000
LandWatch [Member]
Business Acquisition [Line Items]
Goodwill acquired			$ 15,000
The Screening Pros, LLC [Member]
Business Acquisition [Line Items]
Goodwill acquired		$ 2,000
ForRent.Com [Member]
Business Acquisition [Line Items]
Goodwill acquired	$ 267,000

INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	3 Months Ended
INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$ 313,861	$ 182,892
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	2,173	2,275
Finite-lived intangible assets, accumulated amortization	(2,165)	(2,262)
Finite-lived intangible assets, net	$ 8	13
Weighted-average amortization period	4 years
Acquired building photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 8,935	18,739
Finite-lived intangible assets, accumulated amortization	(8,339)	(18,212)
Finite-lived intangible assets, net	$ 596	527
Weighted-average amortization period	2 years
Acquired database technology
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 93,561	83,469
Finite-lived intangible assets, accumulated amortization	(80,405)	(79,188)
Finite-lived intangible assets, net	$ 13,156	4,281
Weighted-average amortization period	4 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 336,376	225,879
Finite-lived intangible assets, accumulated amortization	(175,426)	(169,157)
Finite-lived intangible assets, net	$ 160,950	56,722
Weighted-average amortization period	10 years
Acquired trade names and other intangible assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 188,856	167,718
Finite-lived intangible assets, accumulated amortization	(49,705)	(46,369)
Finite-lived intangible assets, net	$ 139,151	$ 121,349
Weighted-average amortization period	13 years

LONG-TERM DEBT (Details)			3 Months Ended
LONG-TERM DEBT (Details)	Oct. 19, 2017 USD ($)	Apr. 02, 2014	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Interest expense, debt			$ 1,000,000	$ 3,000,000
Capitalized debt issuance costs			3,990,000		$ 4,208,000
Letter of Credit [Member]
Debt Instrument [Line Items]
Debt outstanding			$ 200,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term of loan		5 years
Revolving Credit Facility [Member] \| 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 750,000,000
Debt issuance costs, line of credit arrangements, gross	4,000,000
Unamortized debt issuance expense	5,000,000
Gain (loss) on extinguishment of debt	$ (4,000,000)
Term of loan	5 years
Covenant compliance, secured leverage ratio	3.50
Covenant compliance, total leverage ratio	4.50
Letter of Credit [Member] \| 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 20,000,000
Interest rate, increase (decrease)	2.00%
Letter of Credit [Member] \| 2017 Credit Agreement [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	1.00%
Letter of Credit [Member] \| 2017 Credit Agreement [Member] \| Federal Funds Rate [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	0.50%
Initial Basis Spread [Member] \| Letter of Credit [Member] \| 2017 Credit Agreement [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	1.25%
Initial Basis Spread One Month LIBOR [Member] \| Letter of Credit [Member] \| 2017 Credit Agreement [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	0.25%

INCOME TAXES (Details)	3 Months Ended
INCOME TAXES (Details)	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate (in percent)	6.00%	37.00%

SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended
SEGMENT REPORTING (Details) $ in Thousands	Mar. 31, 2018 USD ($) operating_segments	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of business segments \| operating_segments	2
Segment Reporting Information, Revenue [Abstract]
Revenues	$ 273,718	$ 226,553
Reconciliation of EBITDA to net income (loss) [Abstract]
Net income	52,231	22,130
Amortization of acquired intangible assets in cost of revenues	4,608	6,119
Amortization of acquired intangible assets in operating expenses	5,803	4,774
Depreciation and other amortization	6,572	6,405
Interest and other income	(2,987)	(429)
Interest and other expense	690	2,686
Income tax expense	3,511	13,275
EBITDA	70,428	54,960
Intersegment Revenue [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	13	11
Operating Segments [Member] \| North America [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	264,795	219,341
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA	71,055	54,433
Operating Segments [Member] \| International [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	8,936	7,223
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA	(627)	527
Operating Segments [Member] \| International [Member] \| External Customers [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	8,923	7,212
Intersegment Eliminations [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	$ (13)	$ (11)

SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Property and equipment, net	$ 83,401	$ 84,496
Goodwill	1,551,248	1,283,457	$ 1,254,866
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,095,100	2,873,441
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	294,136	222,191
North America [Member]
Segment Reporting Information [Line Items]
Goodwill	1,520,214	1,253,494	1,227,777
International [Member]
Segment Reporting Information [Line Items]
Goodwill	31,034	29,963	$ 27,089
Operating Segments [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	83,401	84,496
Goodwill	1,551,248	1,283,457
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,095,100	2,873,441
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	294,136	222,191
Operating Segments [Member] \| North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	78,722	79,736
Goodwill	1,520,214	1,253,494
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,037,575	2,816,156
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	274,776	201,831
Operating Segments [Member] \| International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	4,679	4,760
Goodwill	31,034	29,963
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	57,525	57,285
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ 19,360	$ 20,360