Investments, Debt and Equity Securities [Abstract] INVESTMENTS Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Commitments and Contingencies Disclosure [Abstract] Future minimum lease payments Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Apartment Finder [Member] Apartment Finder [Member] Apartment Finder [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Schedule of assets acquired and liabilities assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments [Member] Operating Segments [Member] Intersegment Eliminations [Member] Intersegment Eliminations [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] North America [Member] North America [Member] International [Member] Europe [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Property and equipment, net Property, Plant and Equipment, Net Goodwill Goodwill Reconciliation of operating segment assets to total assets [Abstract] Segment Reconciliation [Abstract] Total assets Assets Reconciliation of operating segment liabilities to total liabilities [Abstract] Segment Reporting Information, Additional Information [Abstract] Total liabilities Liabilities Earnings Per Share [Abstract] Net Income (Loss) Per Share Earnings Per Share, Policy [Policy Text Block] Goodwill [Abstract] GOODWILL Goodwill Disclosure [Text Block] Summarized information by operating segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Reconciliation of net income (loss) to EBITDA Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block] Summarized information by operating segment, assets and liabilities Reconciliation of Assets from Segment to Consolidated [Table Text Block] Revenues by services Revenue from External Customers by Products and Services [Table Text Block] Operating Leases Rent Expense [Abstract] Operating Leases, Rent Expense, Net [Abstract] Rent expense for operating leases Operating Leases, Rent Expense, Net Future Minimum Lease Payments [Abstract] Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] 2018 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2019 Operating Leases, Future Minimum Payments, Due in Two Years 2020 Operating Leases, Future Minimum Payments, Due in Three Years 2021 Operating Leases, Future Minimum Payments, Due in Four Years 2022 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total future minimum lease payments Operating Leases, Future Minimum Payments Due Intangible Assets, Net (Excluding Goodwill) [Abstract] Schedule of acquired finite-lived intangible assets by major class Schedule of Acquired Finite-Lived Intangible Assets by Major Class [Table Text Block] INTANGIBLE ASSETS Intangible Assets Disclosure [Text Block] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Public Float Entity Public Float Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Leasehold Improvements [Member] Leasehold Improvements [Member] Furniture and Office Equipment [Member] Furniture and Fixtures [Member] Vehicles [Member] Vehicles [Member] Computer Hardware and Software [Member] Computer Equipment [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Property and equipment, estimated useful life (in years) Property, Plant and Equipment, Estimated Useful Lives Property and equipment, estimated useful life (in years) Property, Plant and Equipment, Useful Life Stockholders' Equity Note [Abstract] Preferred stock Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Preferred stock authorized for issuance (in shares) Preferred Stock, Shares Authorized Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Common stock Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common stock authorized for issuance (in shares) Common Stock, Shares Authorized Common stock par value (in dollars per share) Common Stock, Par or Stated Value Per Share Equity offering Proceeds from (Repurchase of) Equity [Abstract] Equity offering in period, new shares (in shares) Stock Issued During Period, Shares, New Issues New shares issued, price per share (in dollars per share) Shares Issued, Price Per Share Proceeds from equity offering, net of transaction costs Proceeds from Issuance of Common Stock Payments for underwriting expense Payments for Underwriting Expense Selected Quarterly Financial Information [Abstract] QUARTERLY RESULTS Quarterly Financial Information [Text Block] Income Tax Disclosure [Abstract] Current: Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Foreign Current Foreign Tax Expense (Benefit) Total current Current Income Tax Expense (Benefit) Deferred: Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Foreign Deferred Foreign Income Tax Expense (Benefit) Total deferred Deferred Other Tax Expense (Benefit) Income tax expense, net Income Tax Expense (Benefit) Income Statement [Abstract] Revenues Revenues Cost of revenues Cost of Revenue Gross profit Gross Profit Operating expenses: Operating Expenses [Abstract] Selling and marketing (excluding customer base amortization) Selling and Marketing Expense Software development Research and Development Expense, Software (Excluding Acquired in Process Cost) General and administrative General and Administrative Expense Customer base amortization Amortization of Intangible Assets Total operating expenses Operating Expenses Income from operations Operating Income (Loss) Interest and other income Other Nonoperating Income Interest and other expense Interest Expense Loss on debt extinguishment Gain (Loss) on Extinguishment of Debt Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax expense Net income (loss) Net Income (Loss) Attributable to Parent Net income (loss) per share — basic (in dollars per share) Earnings Per Share, Basic Net income (loss) per share — diluted (in dollars per share) Earnings Per Share, Diluted Weighted average outstanding shares — basic (in shares) Weighted Average Number of Shares Outstanding, Basic Weighted average outstanding shares — diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Organization, Consolidation and Presentation of Financial Statements [Abstract] ORGANIZATION Nature of Operations [Text Block] Furniture, Office Equipment and Vehicles [Member] Property and equipment, gross Property, Plant and Equipment, Gross Accumulated depreciation and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Property and equipment, net Depreciation expense for property and equipment Depreciation Calculation of basic and diluted net income (loss) per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of anti-dilutive securities excluded from computation of earnings per share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] INCOME TAXES Income Tax Disclosure [Text Block] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] LoopNet [Member] LoopNet [Member] Acquisition target. Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Capitalized Product Development Costs [Member] Software and Software Development Costs [Member] Acquired Building Photography [Member] Building Photography [Member] Intangible assets related to building photography that has been capitalized and then amortized over its useful life. Acquired Database Technology [Member] Developed Technology Rights [Member] Acquired Customer Base [Member] Customer Contracts [Member] Acquired Trade Names and Other Intangible Assets [Member] Trade Names [Member] Acquired Finite-Lived Intangible Assets [Line Items] Acquired Finite-Lived Intangible Assets [Line Items] Finite-lived intangible assets, gross Finite-Lived Intangible Assets, Gross Finite-lived intangible assets, accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Finite-lived intangible assets, net Finite-Lived Intangible Assets, Net Weighted-average amortization period (in years} Acquired Finite-lived Intangible Assets, Weighted Average Useful Life Amortization of intangible assets Amortization Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Amortization expense for 2017 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months Amortization expense for 2018 Finite-Lived Intangible Assets, Amortization Expense, Year Two Amortization expense for 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Three Amortization expense for 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Four Amortization expense for 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Five Statement of Comprehensive Income [Abstract] Net income (loss) Other comprehensive loss, net of tax Other Comprehensive Income (Loss), Net of Tax [Abstract] Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Net decrease in unrealized loss on investments Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Reclassification adjustment for realized gains on investments included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Total other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Total comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Valuation and Qualifying Accounts [Abstract] Balance at beginning of year Valuation Allowances and Reserves, Balance Charged to expense Valuation Allowances and Reserves, Additions for Charges to Cost and Expense Charged to other accounts Valuation Allowances and Reserves, Additions for Charges to Other Accounts Write-offs, net of recoveries Valuation Allowances and Reserves, Deductions Balance at end of year Compensation Related Costs [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Domain] Retirement Plan Sponsor Location [Domain] Foreign Plan [Member] Foreign Plan [Member] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Units (RSUs) [Member] Restricted Stock Units (RSUs) [Member] Stock Options [Member] Employee Stock Option [Member] Restricted Stock [Member] Restricted Stock [Member] Restricted Stock Awards — without Market Condition Performance Based RSAs Without Market Condition [Member] Performance Based RSAs Without Market Condition [Member] Restricted Stock Awards — with Market Condition Performance-based RSAs - with Market Condition [Member] Shares or units awarded to employees for meeting certain performance and market targets. Performance Shares [Member] Performance Shares [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] CoStar Group, Inc. 2016 Stock Incentive Plan [Member] CoStar Group, Inc. 2016 Stock Incentive Plan [Member] CoStar Group, Inc. 2016 Stock Incentive Plan [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member] Employee stock incentive plan name. Exercise Price Range [Axis] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] Options Outstanding 1 [Member] Options Outstanding 1 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 2 [Member] Options Outstanding 2 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 3 [Member] Options Outstanding 3 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 4 [Member] Options Outstanding 4 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 5 [Member] Options Outstanding 5 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 6 [Member] Options Outstanding 6 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 7 [Member] Options Outstanding 7 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 8 [Member] Options Outstanding 8 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Outstanding 9 [Member] Options Outstanding 9 [Member] The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. Options Exercisable 1 [Member] Options Exercisable 1 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 2 [Member] Options Exercisable 2 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 3 [Member] Options Exercisable 3 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 4 [Member] Options Exercisable 4 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 5 [Member] Options Exercisable 5 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 6 [Member] Options Exercisable 6 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 7 [Member] Options Exercisable 7 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 8 [Member] Options Exercisable 8 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Options Exercisable 9 [Member] Options Exercisable 9 [Member] The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Shares available for future grant under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Vesting period of options and restricted stock grants Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Shares of common stock authorized for issuance under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Compensation expense Allocated Share-based Compensation Expense Compensation cost expected to be recognized in future years Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted-average-period expected to recognize the unrecognized compensation cost (in years) Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Share-based compensation arrangement, award vesting rights, percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Performance service period Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding at beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Canceled or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding at end of period (in shares) Exercisable at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Outstanding at beginning of period, (in dollars per share) Sharebased Compensation Shares Outstanding Under Stock Option Plans Exercise Price Range The minimum and maximm exercise price range as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices. Granted (in dollars per share) Sharebased Compensation Shares Granted Under Stock Option Plans Exercise Price Range The range of exercise prices for purposes of disclosing stock option granted and other required information pertaining to stock option plans in the customized range. Exercised (in dollars per share) Sharebased Compensation Shares Exercised Under Stock Option Plans Exercise Price Range The range of exercise prices for purposes of disclosing stock option exercised and other required information pertaining to stock option plans in the customized range. Canceled or expired (in dollars per share) Sharebased Compensation Shares Canceled or Expired Under Stock Option Plans Exercise Price Range The range of exercise prices for purposes of disclosing stock option canceled or expired and other required information pertaining to stock option plans in the customized range. Outstanding at end of period, (in dollars per share) Exercisable at end of period (in dollars per share) Exercisable at end of period The range of exercise prices for purposes of disclosing stock option exercisable at the end of the period and other required information pertaining to stock option plans in the customized range. Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Weighted-average exercise price, outstanding at beginning of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Weighted-average exercise price, granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Weighted-average exercise price, exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Weighted-average exercise price, canceled or expired (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Weighted-average exercise price, outstanding at end of period (in dollars per share) Weighted-average exercise price, exercisable at end of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Weighted-average remaining contract life of options outstanding at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Weighted-average remaining contract life of options exercisable at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Aggregate intrinsic value of options outstanding at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Aggregate intrinsic value of options exercisable at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Aggregate intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Weighted-average grant date fair value of each option granted during the period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Dividend yield (in hundredths) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected volatility (in hundredths) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Risk-free interest rate (in hundredths) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected life (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Range of exercise price, minimum, (in dollars per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit Range of exercise price, maximum, (in dollars per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract] Number of shares (in shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options Weighted-average remaining contractual life (in years) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term Weighted- average exercise price (in dollars per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price Options Exercisable [Abstract] Options Exercisable [Abstract] Number of shares (in shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options Weighted-average exercise price (in dollars per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Unvested restricted stock at beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Canceled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Unvested restricted stock at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] Unvested restricted stock at beginning of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Canceled (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Unvested restricted stock at end of period (in dollars per share) Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Capitalized debt issuance costs, net Debt Issuance Costs, Net Amortization of debt issuance costs Amortization of Debt Issuance Costs Cash and Cash Equivalents [Abstract] Number of customers accounting for more than 5% of revenue (in customers) Concentration Risk, Customer Maximum original maturity of highly liquid investments to be considered cash equivalents Additional Cash and Cash Equivalent Related Text Investments Investment, Policy [Policy Text Block] Schedule of goodwill Schedule of Goodwill [Table Text Block] Number of business segments (in segments) Number of Operating Segments Term of subscription-based license agreements (in years) Subscription-Based Licensing Agreement Minimum Term Years Subscription-Based Licensing Agreement Minimum Term Years Finite-Lived Intangible Assets by Major Class [Axis] Percentage of outstanding stock acquired Business Acquisition, Percentage of Voting Interests Acquired Payments to acquire businesses Payments to Acquire Businesses, Gross Closing purchase price adjustments Payments for Previous Acquisition Post closing purchase price adjustments Goodwill, Purchase Accounting Adjustments Number of distinct intangible assets Number of Distinct Intangible Asset The number of distinct intangible assets identified. Estimated useful life of acquired assets Goodwill tax deductible amount Business Acquisition, Goodwill, Expected Tax Deductible Amount Quarterly Financial Information [Table Text Block] Quarterly Financial Information [Table Text Block] Scheduled maturities of investments classified as available-for-sale Investments Classified by Contractual Maturity Date [Table Text Block] Schedule of available for sale securities reconciliation Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Schedule of unrealized loss on investments Schedule of Unrealized Loss on Investments [Table Text Block] SEGMENT REPORTING Segment Reporting Disclosure [Text Block] STOCKHOLDERS' EQUITY Stockholders' Equity Note Disclosure [Text Block] Unrecognized Tax Benefits [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Unrecognized tax benefits beginning balance Unrecognized Tax Benefits Increase for current year tax positions Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Increase for prior year tax positions Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Decrease for prior period tax positions Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Expiration of the statute of limitation for assessment of taxes Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Unrecognized tax benefits ending balance Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods Unrecognized Tax Benefits that Would Impact Effective Tax Rate Interest and penalties on income taxes recognized Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Interest and penalties on income taxes reversed Unrecognized Tax Benefits Reversal Of Interest and Penalties Amount of reversal of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return. Interest and penalties accrued on income taxes Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Accounts receivable, net of allowance for doubtful accounts of approximately $6,469 and $6,344 as of December 31, 2017 and 2016, respectively Accounts Receivable, Net, Current Income tax receivable Income Taxes Receivable, Current Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Total current assets Assets, Current Long-term investments Long-term Investments Deferred income taxes, net Deferred Income Tax Assets, Net Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Deposits and other assets Deposits Assets, Noncurrent Income tax receivable Income Taxes Receivable, Noncurrent Total assets LIABILITIES AND STOCKHOLDERS’ EQUITY Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Current portion of long-term debt Long-term Debt, Current Maturities Accounts payable Accounts Payable, Current Accrued wages and commissions Employee-related Liabilities, Current Accrued expenses Accrued Liabilities, Current Deferred gain on the sale of building Deferred gain on the sale of building, current The current portion of the amount of gain on the sale of property that does not qualify for gain recognition as of the balance sheet date. Income taxes payable Accrued Income Taxes, Current Deferred rent Deferred Rent Credit, Current Deferred revenue Deferred Revenue, Current Total current liabilities Liabilities, Current Long-term debt, less current portion Long-term Debt, Excluding Current Maturities Deferred gain on the sale of building Deferred Gain on Sale of Property Deferred rent Deferred Rent Credit, Noncurrent Deferred income taxes, net Deferred Income Tax Liabilities, Net Income taxes payable Accrued Income Taxes, Noncurrent Total liabilities Commitments and contingencies (Note 10) Commitments and Contingencies Stockholders’ equity: Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Preferred stock, $0.01 par value; 2,000 shares authorized; none outstanding Preferred Stock, Value, Issued Common stock, $0.01 par value; 60,000 shares authorized; 36,107 and 32,606 issued and outstanding as of December 31, 2017 and 2016, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total stockholders’ equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders’ equity Liabilities and Equity Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Statement [Line Items] Statement [Line Items] Allowance for doubtful accounts Allowance for Doubtful Accounts Receivable, Current Preferred stock, shares authorized (in shares) Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Common stock, shares authorized (in shares) Common stock, shares issued, (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Accounting Policies [Abstract] Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Material gains or losses from foreign currency exchange transactions Foreign Currency Transaction Gain (Loss), before Tax Foreign currency translation adjustment Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Net unrealized loss on investments, net of tax Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Total accumulated other comprehensive loss Reclassifications out of accumulated other comprehensive loss Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax ACQUISITIONS Business Combination Disclosure [Text Block] Income Tax Contingency [Table] Income Tax Contingency [Table] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Domain] Income Tax Authority, Name [Domain] Tax Period [Axis] Tax Period [Axis] Tax Period [Domain] Tax Period [Domain] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit) Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense Tax Cuts And Jobs Act Of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense (benefit) Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Transition Tax For Accumulated Foreign Earnings, Provisional Income Tax Expense Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Transition Tax For Accumulated Foreign Earnings, Provisional Income Tax Expense Tax Cuts And Jobs Act Of 2017, incomplete accounting, change in tax rate, provisional income tax expense (benefit) Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Provisional Income Tax Expense Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Provisional Income Tax Expense Subsequent Events [Abstract] SUBSEQUENT EVENTS Subsequent Events [Text Block] Fair Value Disclosures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Asset Class [Domain] Auction Rate Securities [Member] Auction Rate Securities [Member] Fair Value by Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Money Market Funds [Member] Money Market Funds [Member] Commercial Paper [Member] Commercial Paper [Member] Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets: Assets, Fair Value Disclosure [Abstract] Total assets measured at fair value Assets, Fair Value Disclosure Unobservable inputs assets (level 3) [Roll forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Decrease in unrealized loss included in accumulated other comprehensive loss Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Ending balance Auction rate securities variable rate debt instruments interest rate reset period Auction Rate Securities Variable Rate Debt Instruments Interest Rate Reset Period Auction rate securities are variable rate investments that have the interest rate on the security set to the market rate on a contractual basis, this element defines the contractual basis period for the security market change in the interest rate. The minimum contractual maturities on the underlying securities involved in the auction rate securities (in years) The Minimum Contractual Maturities On Underlying Securities Involved In Auction Rate Securities This element represents the underlying maturity dates of the securities the auction rate security is based upon. Par value of company held auction rate securities Available-for-sale Securities, Failed Auction, Value Discount rate (in percent) Fair Value Inputs, Discount Rate Temporary impairment of the auction rates security investments Statement of Stockholders' Equity [Abstract] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock Common Stock [Member] Additional Paid-In Capital Additional Paid-in Capital [Member] Accumulated Other Comprehensive Loss AOCI Attributable to Parent [Member] Retained Earnings Retained Earnings [Member] Balance (in shares) Shares, Outstanding Balance Cumulative effect of adoption of new accounting standard Cumulative Effect of New Accounting Principle in Period of Adoption Net income (loss) Other comprehensive loss Exercise of stock options (in shares) Exercise of stock options Stock Issued During Period, Value, Stock Options Exercised Restricted stock grants (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Gross Restricted stock grants Stock Issued During Period, Value, Restricted Stock Award, Gross Restricted stock grants adjustments Adjustments to Additional Paid in Capital, Other Restricted stock grants surrendered (in shares) Shares Paid for Tax Withholding for Share Based Compensation Restricted stock grants surrendered Adjustments Related to Tax Withholding for Share-based Compensation Stock compensation expense, net of forfeitures Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Stock issued for equity offering (in shares) Stock issued for equity offering Stock Issued During Period, Value, New Issues Employee stock purchase plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Excess tax benefit from stock-based compensation Deferred Tax Expense from Stock Options Exercised Balance (in shares) Balance Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities, Major Types of Debt and Equity Securities [Axis] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Available-for-sale Securities Reconciliation [Abstract] Available-for-sale Securities [Abstract] Amortized cost Available-for-sale Securities, Amortized Cost Basis Gross unrealized gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross unrealized losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities Available-for-sale securities, unrealized loss positions Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] Continuous unrealized loss position, 12 months or more, aggregated fair value Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Continuous unrealized loss position, 12 months or more, gross unrealized losses Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Continuous unrealized loss position, less than 12 months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] Summary of fair value hierarchy for the company's financial assets and liabilities measured at fair value on a recurring basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Summary of changes in the fair value of the company's level 3 assets Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Available-for-sale securities, gross realized gains Available-for-sale Securities, Gross Realized Gains Proceeds from sales of available-for-sale securities Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds Available-for-sale securities, gross realized losses Available-for-sale Securities, Gross Realized Losses Debt Maturities Fair Value [Abstract] Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] 2017 Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 2019 — 2022 Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value 2023 — 2027 Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value 2028 and thereafter Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Available-for-sale investments Commitments and contingencies Liability Reserve Estimate, Policy [Policy Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Acquired Database Technology [Member] Database Rights [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Finite-lived intangible asset, useful life (in years) Finite-Lived Intangible Asset, Useful Life Schedule II - Valuation and Qualifying Accounts Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Line of credit facility, amount outstanding Long-term Line of Credit Interest expense, debt Interest Expense, Debt Interest paid Interest Paid Debt issuance costs, net Revenue from External Customers by Products and Services [Table] Revenue from External Customers by Products and Services [Table] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] CoStar Suite [Member] CoStar Suite [Member] CoStar Suite [Member] Information services [Member] Information services [Member] Information services [Member] Multifamily [Member] Multifamily Online Marketplace [Member] Multifamily Online Marketplace [Member] Commercial property and land [Member] Commercial property and land [Member] Commercial property and land [Member] Revenue from External Customer [Line Items] Revenue from External Customer [Line Items] Stock-Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] Tax Years December 31, 2013 Through December 31, 2017 [Member] Tax Years December 31, 2013 Through December 31, 2017 [Member] Tax Years December 31, 2013 Through December 31, 2017 [Member] Foreign Country [Member] Foreign Tax Authority [Member] Domestic Country [Member] Domestic Tax Authority [Member] State and Local Jurisdiction [Member] State and Local Jurisdiction [Member] Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Change in valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Net operating loss carryforward Operating Loss Carryforwards Operating loss carryforwards, first expiration date Operating Loss Carryforwards, Expiration Date Income tax credit carryforward Tax Credit Carryforward, Amount Cash tax benefits resulting in net operating loss carryforward Realized Tax Benefits Related To Operating Loss Carryforward Amount of cash benefit realized from tax operating losses. An unrecognized tax benefit is presented in the financial statements as a reduction to a deferred tax asset. except as follows. To the extent a operating loss carryforward is not available at the reporting date under the tax law of the applicable jurisdiction to settle any additional income taxes that would result from the disallowance of a tax position, the unrecognized tax benefit is be presented in the financial statements as a liability and is not be combined with deferred tax assets. Research credits Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount Income from U.S. sources Income (Loss) from Continuing Operations before Income Taxes, Domestic Income from foreign sources Income (Loss) from Continuing Operations before Income Taxes, Foreign Income taxes paid Income Taxes Paid Intersegment Revenue [Member] Segment Reconciling Items [Member] Subsegments [Axis] Subsegments [Axis] Subsegments [Domain] Subsegments [Domain] External Customers [Member] External Customers [Member] A component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. Amount of revenue from external customers for the reportable segment. Segment Reporting Information, Revenue [Abstract] Segment Reporting Information, Revenue for Reportable Segment [Abstract] Reconciliation of EBITDA to net income [Abstract] Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract] Amortization of acquired intangible assets in cost of revenues Cost of Goods Sold, Amortization Amortization of acquired intangible assets in operating expenses Amortization Of Intangible Assets, Net Of Adjustments Amortization Of Intangible Assets, Net Of Adjustments Depreciation and other amortization Other Depreciation and Amortization Interest and other income Interest and other expense Loss on debt extinguishment Income tax expense, net EBITDA Net income (loss) before interest, income taxes, depreciation and amortization (“EBITDA”) Net income (loss) before interest, income taxes, depreciation and amortization (“EBITDA”) Net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”) Net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”) Net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”) Term loan facility Long-term Debt, Gross Total debt Long-term Debt Current maturities of long-term debt Other Long-term Debt, Current Current debt issuance costs, net Debt Issuance Costs, Current, Net Total long-term debt, less current portion Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs, by Report Line [Axis] Income Statement Location [Axis] Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Report Line [Domain] Income Statement Location [Domain] Cost of revenues Cost of Sales [Member] Selling and marketing Selling and Marketing Expense [Member] Software development Software Development [Member] General and administrative General and Administrative Expense [Member] Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] Stock-Based Compensation Expense [Abstract] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Net cash proceeds from the exercise of stock options and ESPP Proceeds From Exercise Of Stock Options and ESPP The cash inflow associated with the amount received from holders exercising their stock options and participants of employee stock purchase plans. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. Cash and cash equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Accounts receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Intangible assets, other than goodwill Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Deferred income taxes, net Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets Other assets and liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Fair value of identifiable net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Basis of Presentation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Cost of Revenues Cost of Sales, Policy [Policy Text Block] Foreign Currency Translation Foreign Currency Transactions and Translations Policy [Policy Text Block] Advertising Costs Advertising Costs, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Concentration of Credit Risk and Financial Instruments Concentration Risk, Credit Risk, Policy [Policy Text Block] Accounts Receivable, Net of Allowance for Doubtful Accounts Trade and Other Accounts Receivable, Policy [Policy Text Block] Property and Equipment, Net Property, Plant and Equipment, Policy [Policy Text Block] Goodwill and Intangible Assets Goodwill and Intangible Assets, Policy [Policy Text Block] Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Debt Issuance Costs Debt, Policy [Policy Text Block] Business Combinations Business Combinations Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component [Domain] Variable Rate Component [Domain] [Domain] for Variable Rate Component [Axis] Initial Basis Spread [Member] Initial Basis Spread [Member] Initial Basis Spread [Member] Initial Basis Spread One Month LIBOR [Member] Initial Basis Spread One Month LIBOR [Member] Initial Basis Spread One Month LIBOR [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Letter of Credit [Member] Letter of Credit [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 2017 Credit Agreement [Member] 2017 Credit Agreement [Member] 2017 Credit Agreement [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Federal Funds Effective Swap Rate [Member] Federal Funds Effective Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Term of loan Debt Instrument, Term Debt issuance costs, line of credit Debt Issuance Costs, Line of Credit Arrangements, Gross Unamortized debt issuance expense Unamortized Debt Issuance Expense Loss on extinguishment of debt Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate Interest rate increase (in case of default) Debt Instrument, Interest Rate, Increase (Decrease) Line of credit, covenant compliance, Secured Leverage Ratio Line Of Credit Facility, Covenant Compliance, Secured Leverage Ratio Line Of Credit Facility, Covenant Compliance, Secured Leverage Ratio Line of credit, covenant compliance, Total Leverage Ratio Line Of Credit Facility, Covenant Compliance, Total Leverage Ratio Line Of Credit Facility, Covenant Compliance, Total Leverage Ratio Maximum first lien secured leverage ratio after eight quarters after closing date Maximum first lien secured leverage ratio after eight quarters after closing date Maximum first lien secured leverage ratio after eight quarters after closing date. Maximum total leverage ratio after eight fiscal quarters Maximum total leverage ratio after eight fiscal quarters Maximum total leverage ratio after eight fiscal quarters New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Retained Earnings [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member] Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member] Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member] Accounting Standards Update 2014-09 [Member] Accounting Standards Update 2014-09 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Excess Tax Benefit from Share-based Compensation, Operating Activities Excess Tax Benefit from Share-based Compensation, Operating Activities Deferred Sales Commission Deferred Sales Commission Deferred Sales Commission, Amortization Period Deferred Sales Commission, Amortization Period Deferred Sales Commission, Amortization Period Long-term Debt, Current and Noncurrent [Abstract] Components of long-term debt Schedule of Long-term Debt Instruments [Table Text Block] Advertising costs Advertising Expense Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income (loss) to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Amortization of debt issuance costs Amortization of Debt Issuance Costs and Discounts Impairment loss Impairment of Intangible Assets, Finite-lived Loss on disposal of property and equipment Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down Realized gain on investments Gain (Loss) on Investments, Excluding Other than Temporary Impairments Stock-based compensation expense Share-based Compensation Deferred income tax (benefit) expense, net Deferred Income Tax Expense (Benefit) Bad debt expense Provision for Doubtful Accounts Changes in operating assets and liabilities, net of acquisitions: Increase (Decrease) in Operating Capital [Abstract] Accounts receivable Increase (Decrease) in Accounts Receivable Income taxes payable Increase (Decrease) in Income Taxes Payable, Net of Income Taxes Receivable Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Income tax receivable Increase (Decrease) in Income Taxes Receivable Deposits and other assets Increase (Decrease) in Deposits Accounts payable and other liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Deferred revenue Increase (Decrease) in Deferred Revenue Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from sale and settlement of investments Proceeds from Sale, Maturity and Collection of Investments Purchases of property and equipment and other assets Payments to Acquire Productive Assets Acquisitions, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Payments of long-term debt Repayments of Long-term Debt Payments of debt issuance costs Payments of Debt Issuance Costs Repurchase of restricted stock to satisfy tax withholding obligations Payments Related to Tax Withholding for Share-based Compensation Proceeds from exercise of stock options and employee stock purchase plan Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Effect of foreign currency exchange rates on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents Net increase (decrease) in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of year Cash and cash equivalents at end of year Deferred tax assets: Deferred Tax Assets, Gross [Abstract] Reserve for bad debts Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts Accrued compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation Stock compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Net operating losses Deferred Tax Assets, Operating Loss Carryforwards Accrued reserve and other Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves Unrealized loss on securities Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross Deferred rent Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Deferred Rent Deferred gain on the sale of building Deferred Tax Assets, Deferred Gain on Sale Leaseback Transaction Total deferred tax assets, prior to valuation allowance Deferred Tax Assets, Gross Valuation allowance Deferred Tax Assets, Valuation Allowance Total deferred tax assets, net of valuation allowance Deferred Tax Assets, Net of Valuation Allowance Deferred tax liabilities: Deferred Tax Liabilities, Gross [Abstract] Prepaids Deferred Tax Liabilities, Prepaid Expenses Depreciation Deferred Tax Liabilities, Property, Plant and Equipment Intangibles Deferred Tax Liabilities, Goodwill and Intangible Assets Total deferred tax liabilities Deferred Tax Liabilities, Gross Net deferred tax assets (liabilities) Deferred Tax Liabilities, Net PROPERTY AND EQUIPMENT Property, Plant and Equipment Disclosure [Text Block] EMPLOYEE BENEFIT PLANS Disclosure of Compensation Related Costs, Share-based Payments [Text Block] FAIR VALUE Fair Value Disclosures [Text Block] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Schedule of option activity Share-based Compensation, Stock Options, Activity [Table Text Block] Fair value assumption for options granted Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summarized information regarding options outstanding Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block] Unvested restricted stock awards activity Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] NET INCOME (LOSS) PER SHARE Earnings Per Share [Text Block] Federal Funds Rate [Member] Federal Funds Rate [Member] Federal Funds Rate [Member] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] CoStar Group [Member] Parent Company [Member] Term Loan [Member] Long-term Debt [Member] Swingline Loan [Member] Swingline Loan [Member] Swingline Loan [Member] Notes Payable to Banks [Member] Notes Payable to Banks [Member] Debt instrument borrowing capacity Debt Instrument, Unused Borrowing Capacity, Amount Repayments of term loan Repayments of Debt Revolving credit sub-facility for swing-line loans and issuances of letters of credit Long-term debt Annual amortization, first year after closing (in percent) Percentage of loan amortization in year one Percentage of loan amortization in year one. Annual amortization, second year after closing (in percent) Percentage of loan amortization in year two Percentage of loam amortization in year two. Annual amortization, third year after closing (in percent) Percentage of loan amortization in year three Percentage of loam amortization in year three. Annual amortization, fourth year after closing (in percent) Percentage of loan amortization in year four Percentage of loam amortization in year four. Annual amortization, fifth year after closing (in percent) Percentage of loan amortization in year five Percentage of loam amortization in year five. LIBOR period (in months) LIBOR Maturity Period LIBOR Maturity Period Basis spread on variable rate, one month interest period (in percent) Debt Instrument, Basis Spread on Variable Rate, One Month Interest Period Debt Instrument, Basis Spread on Variable Rate, One Month Interest Period Basis spread on variable rate, per annum (in percent) Debt Instrument, Basis Spread on Variable Rate, Per Annum Debt Instrument, Basis Spread on Variable Rate, Per Annum Default interest rate per annum on overdue amounts (in percent) Default Interest Rate Spread Per Annum The interest rate spread added to applicable interest rate of term loans in the case of default by entity. Credit facility, collateral Line of Credit Facility, Collateral Retirement Benefits [Abstract] Defined Contribution Plan [Table] Defined Contribution Plan [Table] Employee Stock Purchase Plan [Member] EmployeeStockPurchasePlan [Member] An Employee Stock Purchase Plan is a tax-efficient means by which employees of a corporation can purchase the corporation's stock. Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Employee 401(k) Plan [Member] Other Pension Plan [Member] Registered Retirement Savings Plan [Member] Supplemental Employee Retirement Plan [Member] Defined Contribution Plan Disclosure [Line Items] Defined Contribution Plan Disclosure [Line Items] Percentage of company match of employee contribution Percentage of company match of employee contribution The percentage of company's matching contribution to employee's contribution to the plan. Maximum percentage of employee total compensation matched by employer (in hundredths) Defined Contribution Plan, Employer Matching Contribution, Percent of Match Company match to employee contributions Defined Contribution Plan, Cost Administrative expense Defined Contribution Plan, Administrative Expense Percentage of purchase price of company's common stock to the market price Percentage of purchase price of company's common stock to the market price The percentage of market price that eligible employees under the plan can purchase company stock. Increase in shares of common stock issued pursuant to stock plan (in shares) IncreaseInSharesOfCommonStockIssuedPursuantToStockPlan The increase in the number of shares of common stock for issuance under the company stock plan. Shares available, employee stock purchase plan Shares Available, Employee Stock Purchase Plan Number of shares available as a result of an employee stock purchase plan. Shares of Company's common stock purchased during the period (in shares) Schedule of components of income tax expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Schedule of deferred tax assets and liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Schedule of effective income tax rate reconciliation Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Schedule of unrecognized tax benefits Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block] Schedule of accumulated other comprehensive loss Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Stock-based compensation expense for stock options and restricted stock Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] Schedule of property and equipment Property, Plant and Equipment [Table Text Block] Numerator: Earnings Per Share Reconciliation [Abstract] Denominator: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Denominator for basic net income (loss) per share — weighted-average outstanding shares Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Stock options and restricted stock (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Denominator for diluted net income (loss) per share — weighted-average outstanding shares Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive securities excluded from computation of earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Operating expenses Income from operations Schedule of Goodwill [Table] Schedule of Goodwill [Table] The Screening Pros, LLC [Member] The Screening Pros, LLC [Member] The Screening Pros, LLC [Member] Independent Distributor Buyout [Member] Independent Distributor Buyout [Member] Independent Distributor Buyout [Member] Thomas Daily [Member] Thomas Daily [Member] Thomas Daily [Member] Westside Rentals [Member] Westside Rentals [Member] Westside Rentals [Member] LandWatch [Member] LandWatch [Member] LandWatch [Member] Goodwill [Line Items] Goodwill [Line Items] Goodwill [Roll Forward] Goodwill [Roll Forward] Goodwill, beginning balance Acquisitions Goodwill, Acquired During Period Effect of foreign currency translation Goodwill, Foreign Currency Translation Gain (Loss) Goodwill, ending balance LONG-TERM DEBT Long-term Debt [Text Block] Subsequent Event [Table] Subsequent Event [Table] ForRent, Division Of DE Holdings, Inc. ForRent, Division Of DE Holdings, Inc. [Member] ForRent, Division Of DE Holdings, Inc. [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Total purchase price Business Combination, Consideration Transferred Cash payment Purchase price, company stock portion Business Combination, Consideration Transferred, Equity Interests Issued and Issuable Effective tax rate reconciliation [Abstract] Effective Income Tax Rate Reconciliation, Percent [Abstract] Expected federal income tax provision at statutory rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount State income taxes, net of federal benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Foreign income taxes, net effect Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount Increase (decrease) in valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Nondeductible compensation Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount Nondeductible transaction costs Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount Meals and entertainment Effective Income Tax Rate Reconciliation, Nondeductible Expense, Meals and Entertainment, Amount Tax rate changes Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Research credits Excess tax benefit Effective Income Tax Rate Reconciliation, Excess Tax Benefit, Amount Effective Income Tax Rate Reconciliation, Excess Tax Benefit, Amount Tax reserves Effective Income Tax Rate Reconciliation, Change In Tax Reserves, Amount Effective Income Tax Rate Reconciliation, Change In Tax Reserves, Amount Other adjustments Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Acquisitions SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block]