Organization, Consolidation and Presentation of Financial Statements [Abstract] ORGANIZATION Nature of Operations [Text Block] Intangibles and Other Assets [Abstract] -- None. No documentation exists for this element. -- Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] 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] 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] Database Rights [Member] Acquired Customer Base [Member] Customer Contracts [Member] Acquired Trade Names and Other [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 Acquired trade names and other Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets 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 2014 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months Amortization expense for 2015 Finite-Lived Intangible Assets, Amortization Expense, Year Two Amortization expense for 2016 Finite-Lived Intangible Assets, Amortization Expense, Year Three Amortization expense for 2017 Finite-Lived Intangible Assets, Amortization Expense, Year Four Amortization expense for 2018 Finite-Lived Intangible Assets, Amortization Expense, Year Five Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Line Items] Business Acquisition [Line Items] Schedule of consideration paid to acquire business Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of assets acquired and liabilities assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Income Tax Disclosure [Abstract] INCOME TAXES Income Tax Disclosure [Text Block] Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Short-term investments Short-term Investments Accounts receivable, less allowance for doubtful accounts of approximately $2,935 and $3,397 as of December 31, 2012 and 2013, respectively Accounts Receivable, Net, Current Deferred income taxes, net Deferred Tax Assets, Net of Valuation Allowance, Current Income tax receivable Income Taxes Receivable, Current Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Debt issuance costs, net Deferred Costs, Current Total current assets Assets, Current Long-term investments Long-term Investments Property and equipment, net Property, Plant and Equipment, Net Goodwill Goodwill Intangibles and other assets, net Intangible Assets, Net (Excluding Goodwill) Deposits and other assets Deposits Assets, Noncurrent Debt issuance costs, net Deferred Costs, Noncurrent Total assets 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 Sale of Property Income taxes payable Accrued Income Taxes, 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 the sale of building, noncurrent The noncurrent portion of the amount of gain on the sale of property that does not qualify for gain recognition as of the balance sheet date. Deferred rent Deferred Rent Credit, Noncurrent Deferred income taxes, net Deferred Tax Liabilities, Net, Noncurrent Income taxes payable Accrued Income Taxes, Noncurrent Other long-term liabilities Other Liabilities, Noncurrent Total liabilities Liabilities Commitments and contingencies 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; 28,348 and 28,848 issued and outstanding as of December 31, 2012 and 2013, 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 Long-term Debt, Current and Noncurrent [Abstract] LONG-TERM DEBT Long-term Debt [Text Block] 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, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Common stock par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, shares issued, (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Accounting Policies [Abstract] Summary of Significant Accounting Policies [Table] Summary of Significant Accounting Policies [Table] Acquired Customer Base [Member] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Building [Member] Building [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] Summary of Significant Accounting Policies [Line Items] [Line Items] for Summary of Significant Accounting Policies [Table] Term of subscription-based license agreements (in years) Subscription-Based Licensing Agreement Minimum Term Years Subscription-Based Licensing Agreement Minimum Term Years Material gains or losses from foreign currency exchange transactions Foreign Currency Transaction Gain (Loss), before Tax Reclassifications out of accumulated other comprehensive loss Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Advertising costs Advertising Expense Maximum original maturity of highly liquid investments to be considered cash equivalents (in months) Maximum Original Maturity of Highly Liquid Investments to be Considered Cash Equivalents Maximum Original Maturity of Highly Liquid Investments to be Considered Cash Equivalents Cash held to support letters of credit for security deposits Restricted Cash and Cash Equivalents Short-term investment maturities (in days) Short Term Investments Maturity Term Short Term Investments Maturity Term Available for sale short term investment contractual maturities (in years) Available for sale Short Term Investment Contractual Maturities Available for sale Short Term Investment Contractual Maturities Number of customers accouting for more than 5% of revenue (in customers) Number of Customers Accouting for More Than Five Percent of Revenue Number of Customers Accouting for More Than Five Percent of Revenue Significant customer revenue threshold percent Concentration Risk, Percentage Property and equipment, estimated useful life (in years) Property, Plant and Equipment, Useful Life Finite-lived intangible asset, useful life (in years) Finite-Lived Intangible Asset, Useful Life Amortization expense for capitalized product development costs Amortization of Intangible Assets Number of distinct intangible asset Number of Distinct Intangible Asset The number of distinct intangible assets identified. Capitalized product development costs, net Capitalized debt issuance costs Unamortized Debt Issuance Expense Amortization of debt issuance costs Amortization of Financing Costs Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Debt Maturities Fair Value [Abstract] Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] 2014 Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 2015 — 2018 Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value 2019 — 2023 Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value 2024 and thereafter Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Available-for-sale investments Available-for-sale Securities, Debt Securities Number of business segments (in segments) Number of Operating Segments Basis of Presentation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Reclassifications Reclassification, 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] Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss) [Policy Text Block] Disclosure of accounting policy related to components of accumulated other comprehensive income (loss). Advertising Costs Advertising Costs, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Net Income Per Share Earnings Per Share, Policy [Policy Text Block] Stock-Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Investments Investment, 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 Property, Plant and Equipment, Policy [Policy Text Block] Goodwill, Intangibles and Other Assets Goodwill and Intangible Assets, Policy [Policy Text Block] Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Capitalized Product Development Costs Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Debt Issuance Costs Debt, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Income Statement [Abstract] Revenues Revenues Cost of revenues Cost of Revenue Gross margin Gross Profit Operating expenses: Operating Expenses [Abstract] Selling and marketing Selling and Marketing Expense Software development Research and Development Expense, Software (Excluding Acquired in Process Cost) General and administrative General and Administrative Expense Purchase amortization Total operating expenses Operating Expenses Income from operations Operating Income (Loss) Interest and other income Investment Income, Interest Interest and other expense Interest Expense Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax expense, net Income Tax Expense (Benefit) Net income Net Income (Loss) Attributable to Parent Net income per share — basic (in dollars per share) Earnings Per Share, Basic Net income 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 Stockholders' Equity Note [Abstract] Preferred stock Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Preferred stock authorized for issuance (in shares) Common stock Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common stock authorized for issuance (in shares) Increase in shares authorized (in shares) Increase In Shares Authorized The increase in the number of common shares authorized by an entity's charter and bylaws. 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] 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 Goodwill Business Combination, Recognized Goodwill Acquired The amount of goodwill recognized as of the acquisition date. Acquired customer base Business Acquisition, Purchase Price Allocation, Acquired Customer Base Business Acquisition, Purchase Price Allocation, Acquired Customer Base Acquired database technology Business Acquisition, Purchase Price Allocation, Acquired Database Technology Business Acquisition, Purchase Price Allocation, Acquired Database Technology Deferred income taxes, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets and Liabilities, Net Amount of acquisition cost of a business combination allocated to noncurrent deferred tax assets net of valuation allowance and noncurrent deferred tax liabilities. Other assets and liabilities Business Acquisition, Purchase Price Allocation, Other Assets and Liabilities Amount of current and non-current other assets and other liabilities, net acquired at the acquisition date. Fair value of identifiable net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Commitments and Contingencies Disclosure [Abstract] Future minimum lease payments Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] ACQUISITIONS Business Combination Disclosure [Text Block] Schedule of acquired finite-lived intangible assets by major class Schedule of Acquired Finite-Lived Intangible Assets by Major Class [Table Text Block] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Federal Funds Rate [Member] Federal Funds Rate [Member] Federal Funds Rate [Member] LIBOR [Member] London Interbank Offered Rate (LIBOR) [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] CoStar Group [Member] Parent Company [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Committed Term Loan [Member] Notes Payable to Banks [Member] New Term Loan [Member] Long-term Debt [Member] New Revolving Credit Facility [Member] Revolving Credit Facility [Member] Swingline Loan [Member] Swingline Loan [Member] Swingline Loan [Member] Letter of Credit [Member] Letter of Credit [Member] Debt instrument borrowing capacity Debt Instrument, Unused Borrowing Capacity, Amount Term of loan (in years) Debt Instrument, Term Line of credit facility, amount outstanding Line of Credit Facility, Amount Outstanding Revolving credit sub-facility for swing-line loans Line of Credit Facility, Maximum Borrowing Capacity 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. Basis spread on federal funds rate (in percent) Debt Instrument, Basis Spread on Variable Rate 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. Maximum debt service coverage ratio (in percent) Maximum Debt Service Coverage Ratio Maximum Debt Service Coverage Ratio. Maximum total leverage ratio for fifth through eighth quarters after closing date (in percent) Maximum Total leverage ratio for fifth through eighth quarters after closing date Maximum Total leverage ratio for fifth through eighth quarters after closing date Maximum total leverage ratio after eight fiscal quarters (in percent) Maximum total leverage ratio after eight fiscal quarters Maximum total leverage ratio after eight fiscal quarters Excess cash flow payment maturity period within issuance of financial statements Excess Cash Flow Payment Maturity Period Within Issuance of Financial Statements Excess Cash Flow Payment Maturity Period Within Issuance of Financial Statements First excess cash flow repayment percentage First Excess Cash Flow Repayment, Percentage First Excess Cash Flow Repayment, Percentage First excess cash flow repayment requirement (in percent) First Excess Cash Flow Repayment Requirement First Excess Cash Flow Repayment Requirement Second excess cash flow repayment requirement (in percent) Second Excess Cash Flow Repayment Requirement Second Excess Cash Flow Repayment Requirement Second excess cash flow repayment percentage Second Excess Cash Flow Repayment, Percentage Second Excess Cash Flow Repayment, Percentage Third excess cash flow repayment requirement (in percent) Third Excess Cash Flow Repayment Requirement Third Excess Cash Flow Repayment Requirement Third excess cash flow repayment percentage Third Excess Cash Flow Repayment, Percentage Third Excess Cash Flow Repayment, Percentage Unamortized debt issuance expense Debt Instrument, Unamortized Discount (Premium), Net Debt issuance underwriting fees Debt issuance underwriting fees The expense associated with the underwriting of a new debt issuance by lender. Legal fees associated with the debt issuance Debt Issuance Legal Fees The legal fees associated with the issuance of new debt. Debt interest expense Interest Expense, Debt Interest paid Interest Paid 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] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Performance-Based Restricted Common Stock [Member] Performance-Based Restricted Common Stock [Member] Performance-Based Restricted Common Stock [Member] 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] Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] Stock-Based Compensation Expense [Abstract] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Compensation expense Allocated Share-based Compensation Expense Unrecognized compensation cost expected to be recognized in future years Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized 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. Excess tax benefit from stock-based compensation Deferred Tax Expense from Stock Options Exercised Goodwill [Abstract] -- None. No documentation exists for this element. -- Schedule of Goodwill [Table] Schedule of Goodwill [Table] Statement, Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] United States [Member] United States [Member] A geographical business segment of CoStar Realty Information Group. International [Member] International [Member] A geographical business segment of CoStar Realty Information Group. Goodwill [Line Items] Goodwill [Line Items] Goodwill, period increase (decrease) Goodwill, Period Increase (Decrease) Goodwill [Roll Forward] Goodwill [Roll Forward] Goodwill, beginning balance Acquisitions Goodwill, Acquired During Period Effect of foreign currency translation Goodwill, Translation Adjustments Goodwill, ending balance Property, Plant and Equipment [Abstract] Schedule of Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Leasehold Improvements [Member] Leasehold Improvements [Member] Furniture, Office Equipment and Research Vehicles [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] 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 Valuation and Qualifying Accounts [Abstract] Balance at Beginning of Year Valuation Allowances and Reserves, Balance Charged to Expense Valuation Allowances and Reserves, Charged to Cost and Expense Charged to Other Accounts Valuation Allowances and Reserves, Charged to Other Accounts Write-offs, Net of Recoveries Valuation Allowances and Reserves, Deductions Balance at End of Year Compensation Related Costs [Abstract] EMPLOYEE BENEFIT PLANS Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] 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. Intersegment Revenue [Member] Intersegment Revenue [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 Intersegment Elimination [Member] Intersegment Eliminations [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Corporate allocation US Corporate allocation US Cost incurred for international employees involved in the management and expansion activities of the company's US operating segment. Corporate allocation Corporate allocation Cost incurred for domestic employees involved in the management and expansion activities of the company's International operating segment. Summarized information by operating segment [Abstract] Segment Reporting Information, Additional Information [Abstract] Reconciliation of EBITDA to net income [Abstract] Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract] EBITDA Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest Purchase amortization in cost of revenues Cost of Goods Sold, Amortization Purchase amortization in operating expenses Depreciation and other amortization Depreciation, Depletion and Amortization Interest income Interest expense Income tax expense, net Reconciliation of operating segment assets to total assets [Abstract] Segment Reconciliation [Abstract] Total operating segment assets Segment Reporting Segment Gross Assets The gross assets by segment before intercompany eliminations are considered. Investment in subsidiaries Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Intersegment receivables Accounts Receivable, Related Parties Reconciliation of operating segment liabilities to total liabilities [Abstract] Reconciliation of segment liabilities to total liabilities [Abstract] -- None. No documentation exists for this element. -- Total operating segment liabilities Total segment liabilities Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized in reportable segments. Intersegment payables Accounts Payable, Related Parties Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Stock Options [Member] Employee Stock Option [Member] Restricted Stock [Member] Restricted Stock [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 1998 Stock Incentive Plan [Member] Stock Incentive Plan 1998 [Member] Employee stock incentive plan name. CoStar Group, Inc. 2007 Stock Incentive Plan [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member] Employee stock incentive plan name. Employee Stock Purchase Plan [Member] Employee Stock Purchase Plan [Member] The employee stock purchase plan that allows eligible employees to purchase company stock at a discount. 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 Outstanding 10 [Member] Options Outstanding 10 [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. Options Exercisable 10 [Member] Options Exercisable 10 [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. Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Employee 401(k) Plan [Member] Other Pension Plan, Defined Benefit [Member] Employee Pension Plan, London Office [Member] Foreign Pension Plan, Defined Benefit [Member] 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 Increase in shares of common stock pursuant to amendment to the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Other Share Increase (Decrease) 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 of common stock authorized for issuance under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Cumulative EBITDA required for award shares to vest Cumulative EBITDA Required for Award Shares to Vest Cumulative EBITDA Required for Award Shares to Vest Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters) Number of Consecutive Quarters to Mantain Cumulative EBITDA Required for Award Shares to Vest Number of Consecutive Quarters to Mantain Cumulative EBITDA Required for Award Shares to Vest 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] Outstanding at beginning of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, 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 Exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, 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 Outstanding at end of period (in dollars per share) 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-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 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] -- None. No documentation exists for this element. -- 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) Employee Plans [Abstract] Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract] Percentage of company match of employee contribution (in hundredths) 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 (401k) Defined Contribution Plan, Cost Recognized Administrative expense Defined Contribution Plan, Administrative Expenses Company match to employee contributions (pension) Defined Benefit Plan, Contributions by Employer Percentage of purchase price of Company's common stock to the market price (in hundredths) 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. Shares available for purchase (in shares) 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) Stock Issued During Period, Shares, Employee Stock Purchase Plans 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 Summarized information by operating segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Reconciliation of EBITDA to net income 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] Schedule of components of income tax expense (benefit) 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] Fair Value Disclosures [Abstract] FAIR VALUE Fair Value Disclosures [Text Block] Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation Amortization Amortization of debt issuance costs Amortization of Financing Costs and Discounts Property and equipment write-off Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down Excess tax benefit from stock-based compensation Excess Tax Benefit from Share-based Compensation, Operating Activities Stock-based compensation expense Share-based Compensation Deferred consideration settlement Business acquisition deferred consideration liability reversal The amount of deferred consideration liability from a business acquisition that was reversed. Deferred income tax expense (benefit), net Deferred Income Tax Expense (Benefit) Provision for losses on accounts receivable 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 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 Proceeds from sale of building, net Sale Leaseback Transaction, Net Proceeds 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 provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from long-term debt Proceeds from Issuance of Long-term Debt Payments of long-term debt Repayments of Long-term Debt Payments of debt issuance costs Payments of Debt Issuance Costs Payments of deferred consideration Proceeds from (Payments for) Other Financing Activities Excess tax benefit from stock-based compensation Excess Tax Benefit from Share-based Compensation, Financing Activities Repurchase of restricted stock to satisfy tax withholding obligations Payments Related to Tax Withholding for Share-based Compensation Proceeds from equity offering, net of transaction costs Proceeds from Issuance of Common Stock Proceeds from exercise of stock options and employee stock purchase plan Net cash provided by 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 Schedule of option activity Schedule of 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 unrestricted stock awards activity Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] STOCKHOLDERS' EQUITY Stockholders' Equity Note Disclosure [Text Block] Earnings Per Share [Abstract] NET INCOME PER SHARE Earnings Per Share [Text Block] Statement of Comprehensive Income [Abstract] Other comprehensive income, 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 Total other comprehensive income Other Comprehensive Income (Loss), Net of Tax Total comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Schedule of Goodwill Schedule of Goodwill [Table Text Block] Numerator: Earnings Per Share Reconciliation [Abstract] Denominator: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Denominator for basic net income 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 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 Cash Payments to Acquire Businesses, Gross Equity interest, number of shares (in shares) Business Acquisition, Equity Interest Issued or Issuable, Number of Shares Equity interest, price per share (in dollars per share) Business Acquisition, Share Price Equity interest (1,880,300 shares at $72.89) Business Combination, Consideration Transferred, Equity Interests Issued and Issuable Fair value of total consideration transferred Business Combination, Consideration Transferred 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] Summary of changes in the fair value of the company's level 3 liabilities Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] INVESTMENTS Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] 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 Total Deferred Income Tax Per Year The component of income tax expense for the period representing the increase (decrease) in the entity's deferred tax assets and liabilities pertaining to continuing operations. Total provision for income taxes 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 revenue Deferred Tax Assets, Deferred Income Deferred gain from sale of building Deferred Tax Assets, Deferred Gain on Sale Leaseback Transaction Total deferred tax assets Deferred Tax Assets, Gross 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, Intangible Assets Total deferred tax liabilities Deferred Tax Liabilities, Net Net deferred tax liabilities, prior to valuation allowance Deferred Tax Assets, Net Valuation allowance Deferred Tax Assets, Valuation Allowance Net deferred tax liabilities Deferred Tax Assets, Net of Valuation Allowance Income from U.S. sources Income (Loss) from Continuing Operations before Income Taxes, Domestic Loss from foreign sources Income (Loss) from Continuing Operations before Income Taxes, Foreign 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 Stock compensation Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount Increase 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 Other adjustments Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Income taxes paid Income Taxes Paid Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] 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, Change in 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 Unrecognized Tax Benefits Resulting in Net Operating Loss Carryforward Unrecognized Tax Benefits [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Unrecognized tax benefit beginning balance Unrecognized Tax Benefits Increase for current year tax positions Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Decrease for prior period tax positions Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Increase for prior year tax positions Unrecognized Tax Benefits, Increase 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 benefit ending balance Unrecognized tax benefit 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 accrued on income taxes Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued INTANGIBLES AND OTHER ASSETS Intangible Assets Disclosure [Text Block] Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Foreign currency translation adjustment Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Accumulated 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 Schedule of property, plant and equipment Property, Plant and Equipment [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] Property and equipment estimated useful lives Property Plant and Equipment Estimated Useful Lives [Table Text Block] A tabular disclosure of the estimated useful lives of property, plant and equipment. GOODWILL Goodwill Disclosure [Text Block] COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] Percentage of outstanding stock controlled by company Business Acquisition, Percentage of Voting Interests Acquired Estimated useful life of acquired assets Business combination, recognized goodwill acquired Goodwill tax deductible amount Business Acquisition, Goodwill, Expected Tax Deductible Amount Incurred acquisition-related costs Business Combination, Acquisition Related Costs Term of consent order public comment period Term of Consent Order Public Comment Period Term of Consent Order Public Comment Period Proceeds from sale of Xcelignet Proceeds from Sale of Investment Projects Gain on sale of Xceligent Gain (Loss) on Sale of Other Investments SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] PROPERTY AND EQUIPMENT Property, Plant and Equipment Disclosure [Text Block] Schedule of Maturities of Long-term Debt Schedule of Maturities of Long-term Debt [Table Text Block] Allowance for doubtful accounts and billing adjustments Allowance for doubtful accounts and billing adjustments [Table Text Block] Tabular disclosure of allowances reported. Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities, 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] Government-Sponsored Enterprise Obligations [Member] US Government-sponsored Enterprises Debt Securities [Member] Auction Rate Securities [Member] Auction Rate Securities [Member] Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss Available-for-sale Securities, Gross Realized Gains Available-for-sale Securities, Gross Realized Gains Available-for-sale Securities, Gross Realized Losses Available-for-sale Securities, Gross Realized Losses Available-for-sale Securities Reconciliation [Abstract] Available-for-sale Securities [Abstract] Amortized Cost Available-for-sale Securities, Amortized Cost Basis Gross Unrealized Gains Gross Unrealized Gains for Available For Sale Securities This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities. Gross Unrealized Losses Gross Unrealized Losses for Available For Sale Securities This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities. Available-for-sale securities, unrealized loss positions Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] Aggregate Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Statement of Stockholders' Equity [Abstract] Statement, 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 Income (Loss) Accumulated Other Comprehensive Income (Loss) [Member] Retained Earnings Retained Earnings [Member] Balance (in shares) Shares, Outstanding Balance Net income Foreign currency translation adjustment Net change in unrealized gain (loss) on investments Unrealized Gain (Loss) on Investments 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 surrendered (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Forfeited Restricted stock grants surrendered Stock Issued During Period, Value, Restricted Stock Award, Forfeitures 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 During Period, Shares, New Issues Stock issued for equity offering Stock Issued During Period, Value, New Issues Employee stock purchase plan (in shares) Employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Consideration for LoopNet, Inc. (in shares) Stock Issued During Period, Shares, Acquisitions Consideration for LoopNet, Inc. Stock Issued During Period, Value, Acquisitions Balance (in shares) Balance Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] LandsofAmerica [Member] LandsofAmerica [Member] LandsofAmerica [Member] Reaction Web [Member] Reaction Web [Member] Reaction Web [Member] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Asset Class [Domain] 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] Cash [Member] Cash [Member] Money Market Funds [Member] Money Market Funds [Member] Commercial Paper [Member] Commercial Paper [Member] Fair Value, Assets and Liabilities 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 Auction rate securities upon acquisition Auction Rate Securities Upon Acquisition Auction Rate Securities Upon Acquisition Change in unrealized gain (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 Temporary impairment of the auction rates security investments Liabilities: Liabilities, Fair Value Disclosure [Abstract] Deferred consideration Financial and Nonfinancial Liabilities, Fair Value Disclosure Unobservable inputs liabilities (level 3) [Roll forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning balance Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Accretion Accretion (Amortization) of Discounts and Premiums, Investments Deferred consideration upon acquisition Fair Value Liabilities, Deferred Consideration Upon Acquisition Fair Value Liabilities, Deferred Consideration Upon Acquisition Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Ending balance Business acquisition deferred consideration potential cash payment (undiscounted) Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Discount rate (in percent) Fair Value Inputs, Discount Rate Business acquisition deferred consideration cash payment Business Acquisition Contingent Consideration Cash Payment Amount of cash paid to settle a deferred consideration liability. 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] 2014 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2015 Operating Leases, Future Minimum Payments, Due in Two Years 2016 Operating Leases, Future Minimum Payments, Due in Three Years 2017 Operating Leases, Future Minimum Payments, Due in Four Years 2018 Operating Leases, Future Minimum Payments, Due in Five Years 2019 and thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total future minimum lease payments Operating Leases, Future Minimum Payments Due Litigation Case [Axis] Litigation Case [Axis] Litigation Case [Domain] Litigation Case [Domain] LoopNet Stockholders [Member] LoopNet Stockholders [Member] LoopNet Stockholders [Member] Civix [Member] Civix [Member] Civix [Member] Number of class action lawsuits brought by alleged LoopNet stockholders Loss Contingency, New Claims Filed, Number Loss contingency accrual Loss Contingency Accrual Loss contingency, estimated damages sought Loss Contingency, Damages Sought, Value Estimate of possible loss in excess of amount accrued Loss Contingency, Estimate of Possible Loss Number of minimum of current litigation matters (in claims) Minimum Number of Current Litigation Matters Minimum Number of Current Litigation Matters 2014 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2015 Long-term Debt, Maturities, Repayments of Principal in Year Two 2016 Long-term Debt, Maturities, Repayments of Principal in Year Three 2017 Long-term Debt, Maturities, Repayments of Principal in Year Four Long-term debt, including current maturities Long-term Debt Calculation of basic and diluted net income per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule II - Valuation and Qualifying Accounts Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] SEGMENT REPORTING Segment Reporting Disclosure [Text Block]