Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] 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] Statement [Line Items] Statement [Line Items] Balance (in shares) Shares, Outstanding Balance Stockholders' Equity Attributable to Parent Cumulative effect of adoption of new accounting standard, net of tax Cumulative Effect of New Accounting Principle in Period of Adoption Balance at January 1, 2018 Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Net income (loss) Net Income (Loss) Attributable to Parent Other comprehensive loss Other Comprehensive Income (Loss), Net of Tax Exercise of stock options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period 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 During Period, Shares, New Issues 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 Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation Stock issued for acquisitions (in shares) Stock Issued During Period, Shares, Acquisitions Stock issued for acquisitions Stock Issued During Period, Value, Acquisitions Balance (in shares) Balance Selected Quarterly Financial Information [Abstract] Revenues Revenue from Contract with Customer, Excluding Assessed Tax Cost of revenues Cost of Revenue Gross profit Gross Profit 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 Income Tax Expense (Benefit) Net income 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 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] Segment Reporting [Abstract] Summarized information by operating segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Reconciliation of net income 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] Investments, Debt and Equity Securities [Abstract] 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] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Auction Rate Securities [Member] Auction Rate Securities [Member] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Realized gain on investments Debt Securities, Available-for-sale, Realized Gain Realized loss on investments Debt Securities, Available-for-sale, Realized Loss Amortized Cost Debt Securities, Available-for-sale, Amortized Cost Gross Unrealized Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Fair Value Debt Securities, Available-for-sale Aggregate Fair Value Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer Gross Unrealized Losses Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Goodwill [Abstract] Schedule of Goodwill [Table] Schedule of Goodwill [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Westside Rentals [Member] Westside Rentals [Member] Westside Rentals [Member] LandWatch [Member] LandWatch [Member] LandWatch [Member] The Screening Pros, LLC [Member] The Screening Pros, LLC [Member] The Screening Pros, LLC [Member] ForRent, Division Of DE Holdings, Inc. [Member] ForRent, Division Of DE Holdings, Inc. [Member] ForRent, Division Of DE Holdings, Inc. [Member] Realla Ltd [Member] Realla Ltd [Member] Realla Ltd [Member] Cozy Services Ltd [Member] Cozy Services Ltd [Member] Cozy Services Ltd [Member] Goodwill [Line Items] Goodwill [Line Items] Acquisitions Goodwill, Acquired During Period Goodwill Goodwill Goodwill tax deductible amount Business Acquisition, Goodwill, Expected Tax Deductible Amount Goodwill, impairment loss Goodwill, Impairment Loss Property, Plant and Equipment [Abstract] Schedule of property and equipment Property, Plant and Equipment [Table Text Block] Revenue from Contract with Customer [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] North America [Member] North America [Member] International [Member] International [Member] International [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Information And Analytics [Member] Information And Analytics [Member] Information And Analytics [Member] Online Marketplaces [Member] Online Marketplaces [Member] Online Marketplaces [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] CoStar Suite [Member] CoStar Suite [Member] CoStar Suite [Member] Information services [Member] Information services [Member] Information services [Member] Multifamily Online Marketplace [Member] Multifamily Online Marketplace [Member] Multifamily Online Marketplace [Member] Commercial property and land [Member] Commercial property and land [Member] Commercial property and land [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Total revenues Earnings Per Share [Abstract] 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] 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), Debt Securities, Available-for-sale, Adjustment, after Tax Total accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Reclassifications out of accumulated other comprehensive loss Reclassification from Accumulated Other Comprehensive Income, Current 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 Basis of Presentation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Revenue Recognition Revenue from Contract with Customer [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] 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, 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] Liability Reserve Estimate Liability Reserve Estimate, Policy [Policy Text Block] Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Debt issuance costs, net Debt Issuance Costs, Net Amortization of debt issuance costs Amortization of Debt Issuance Costs Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Number of business segments Number of Operating Segments Segment Reporting Information, Revenue [Abstract] Segment Reporting Information, Revenue for Reportable Segment [Abstract] Reconciliation of EBITDA to net income (loss) [Abstract] Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract] Amortization of acquired intangible assets in cost of revenues Amortization Of Intangible Assets, Cost of Revenue Amortization Of Intangible Assets, Cost of Revenue 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 extinguishment of debt 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”) Statement of Financial Position [Abstract] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] 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 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 (excluding customer base amortization) 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] 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 Expense related to stock options settled Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost INVESTMENTS Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Revenue, Initial Application Period Cumulative Effect Transition [Table] Revenue, Initial Application Period Cumulative Effect Transition [Table] 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 [Member] Accounting Standards Update 2016-09 [Member] Accounting Standards Update 2014-09 [Member] Accounting Standards Update 2014-09 [Member] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Domain] Initial Application Period Cumulative Effect Transition [Domain] Calculated under Revenue Guidance in Effect before Topic 606 [Member] Calculated under Revenue Guidance in Effect before Topic 606 [Member] Difference between Revenue Guidance in Effect before and after Topic 606 [Member] Difference between Revenue Guidance in Effect before and after Topic 606 [Member] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] Revenues Decrease in Revenue Decrease in Revenue Selling and marketing expense Selling and Marketing Expense Net income Earnings per share basic and diluted (usd per share) Earnings Per Share, Basic and Diluted Commitments and Contingencies Disclosure [Abstract] 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] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 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 Goodwill [Roll Forward] Goodwill [Roll Forward] Goodwill, beginning balance Effect of foreign currency translation Goodwill, Foreign Currency Translation Gain (Loss) Goodwill, ending balance New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Accounts receivable, less allowance for doubtful accounts Accounts Receivable, Net, Current Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Deferred commission costs, net Deferred Costs, Noncurrent Deferred revenue Contract with Customer, Liability, Current Deferred income taxes, net Deferred Income Tax Liabilities, Net Retained earnings Retained Earnings (Accumulated Deficit) NET INCOME PER SHARE Earnings Per Share [Text Block] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] 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 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 Impairment of Intangible Assets (Excluding Goodwill) Impairment of Intangible Assets (Excluding Goodwill) 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 2019 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months Amortization expense for 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two Amortization expense for 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three Amortization expense for 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four Amortization expense for 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five Business Combinations [Abstract] ACQUISITIONS Business Combination Disclosure [Text Block] Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Federal Funds Rate [Member] Federal Funds Rate [Member] Federal Funds Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] 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] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] 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] Debt Instrument [Line Items] Debt Instrument [Line Items] Revolving credit sub-facility for swing-line loans and issuances of letters of credit Line of Credit Facility, Maximum Borrowing Capacity Term of facility Line of Credit Facility, Expiration Period Debt issuance costs, line of credit Debt Issuance Costs, Line of Credit Arrangements, Gross Unamortized debt issuance expense Unamortized Debt Issuance Expense Long-term debt Long-term 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 Interest expense, debt Interest Expense, Debt Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] 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 and amortization Depreciation, Depletion and Amortization Amortization of deferred commissions costs Amortization of Deferred Sales Commissions 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 Debt Securities, Realized Gain (Loss) Stock-based compensation expense Share-based Compensation Deferred income taxes, 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 Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Deferred commissions Increase (Decrease) Deferred Commissions Increase (Decrease) Deferred Commissions Income tax receivable Increase (Decrease) in Income Taxes Receivable Accounts payable and other liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Income taxes payable Increase (Decrease) in Income Taxes Payable, Net of Income Taxes Receivable Deferred revenue Increase (Decrease) in Contract with Customer, Liability Deferred revenue Increase (Decrease) in Contract with Customer, Asset Deposits and other assets Increase (Decrease) in Deposits 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 Cash paid for 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 equity offering, net of transaction costs Proceeds from Issuance of Common Stock Proceeds from exercise of stock options and employee stock purchase plan 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. 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 (decrease) increase 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 Carrying Value Cash and cash equivalents at end of year Supplemental cash flow disclosures: Supplemental Cash Flow Information [Abstract] Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes paid Income Taxes Paid Supplemental non-cash investing and financing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Stock issued in connection with acquisition - ForRent Other Significant Noncash Transaction, Value of Consideration Given Consideration owed for acquisitions Noncash or Part Noncash Acquisition, Consideration Owed for Acquisitions Noncash or Part Noncash Acquisition, Consideration Owed for Acquisitions Fair Value Disclosures [Abstract] FAIR VALUE Fair Value Disclosures [Text Block] 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] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Units (RSUs) [Member] Restricted Stock Units (RSUs) [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] 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] 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) 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] QUARTERLY RESULTS OF OPERATIONS Quarterly Financial Information [Text Block] Future minimum lease payments Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Quarterly Financial Information Quarterly Financial Information [Table Text Block] Income Tax Disclosure [Abstract] 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 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 Research and development credits Deferred Tax Assets, Tax Credit Carryforwards 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] Deferred commission costs, net Deferred Tax Liabilities, Accrued Commissions Deferred Tax Liabilities, Accrued Commissions Prepaid expenses Deferred Tax Liabilities, Prepaid Expenses Property and equipment, net Deferred Tax Liabilities, Property, Plant and Equipment Intangible assets, net Deferred Tax Liabilities, Goodwill and Intangible Assets Total deferred tax liabilities Deferred Tax Liabilities, Gross Net deferred tax assets (liabilities) Deferred Tax Liabilities, Net Revenue Business Acquisition, Pro Forma Revenue Net income Business Acquisition, Pro Forma Net Income (Loss) Net income per share - basic (usd per share) Business Acquisition, Pro Forma Earnings Per Share, Basic Net income per share - diluted (usd per share) Business Acquisition, Pro Forma Earnings Per Share, Diluted 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] 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 Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company 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 Organization, Consolidation and Presentation of Financial Statements [Abstract] ORGANIZATION Nature of Operations [Text Block] Stock Options [Member] Employee Stock Option [Member] Performance-based RSAs - with Market Condition [Member] Performance-based RSAs - with Market Condition [Member] Shares or units awarded to employees for meeting certain performance and market targets. 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] Debt Maturities Fair Value [Abstract] Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] 2019 Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value 2020 — 2023 Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value 2024 — 2028 Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value 2029 and thereafter Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Available-for-sale investments Available-for-sale Securities SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Term of subscription-based license agreements (in years) Subscription-Based Licensing Agreement Minimum Term Years Subscription-Based Licensing Agreement Minimum Term Years Advertising costs Advertising Expense Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] 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, Office Equipment and Vehicles [Member] Furniture and Fixtures [Member] Computer Hardware and Software [Member] Computer Equipment [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 Property, Plant and Equipment, Net Depreciation expense for property and equipment Depreciation Property and equipment, net 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 Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Retained Earnings [Member] Accounting Standards Update 2016-02 [Member] Accounting Standards Update 2016-02 [Member] Right-of-Use asset Operating Lease, Right-of-Use Asset Lease liability Operating Lease, Liability EMPLOYEE BENEFIT PLANS Disclosure of Compensation Related Costs, Share-based Payments [Text Block] INCOME TAXES Income Tax Disclosure [Text Block] Exercise Price Range [Axis] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] $54.51 - $78.33 Exercise Range 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. $78.34 - $142.45 Exercise Range 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. $142.46 - $188.22 Exercise Range 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. $188.23 - $197.37 Exercise Range 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. $197.38 - $202.98 Exercise Range 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. $202.99 - $273.52 Exercise Range 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. $273.53 - $342.13 Exercise Range 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. $54.51 - $342.13 Exercise Range 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. 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 Schedule of goodwill Schedule of Goodwill [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Finite-Lived Intangible Assets by Major Class [Axis] Customer Relationships [Member] Customer Relationships [Member] Acquired Database Technology [Member] Database Rights [Member] Acquired trade names and other intangible assets [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Aggregate purchase price Business Combination, Consideration Transferred Cash payment Payments to Acquire Businesses, Gross Investment owned (shares) Investment Owned, Shares Investment Owned, Shares Purchase price, shares issued Business Combination, Consideration Transferred, Equity Interests Issued and Issuable Weighted-average amortization period Goodwill, expected tax deductible amount Indemnification asset Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability State uncertain income tax position liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, State Uncertain Income Tax Position Liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, State Uncertain Income Tax Position Liability Indemnification asset Business Combination, Indemnification Assets, Amount as of Acquisition Date Indemnification asset Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indemnification Asset Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indemnification Asset Transaction costs Business Acquisition, Transaction Costs Employee retention bonus Business Combination, Employee Bonus Compensation for Retention Business Combination, Employee Bonus Compensation for Retention 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 Weighted Average Number of Shares Outstanding, Basic Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Stock options and restricted stock awards (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 Weighted Average Number of Shares Outstanding, Diluted 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] Restricted Stock [Member] Restricted Stock [Member] 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 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, Increase (Decrease), 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 Research credits Effective Income Tax Rate Reconciliation, Tax Credit, Research, 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. 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) Equity offering Proceeds from (Repurchase of) Equity [Abstract] Equity offering in period, new shares (in shares) New shares issued, price per share (in dollars per share) Shares Issued, Price Per Share Payments for underwriting expense Payments for Underwriting Expense Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Amount Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, remaining performance obligation, expected timing of satisfaction, period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Accounts receivable, less allowance of $5,709 and $6,469 as of December 31, 2018 and December 31, 2017, respectively 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] 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 Total current liabilities Liabilities, Current Deferred gain on the sale of building Deferred Gain on Sale of Property Deferred rent Deferred Rent Credit, Noncurrent Income taxes payable Accrued Income Taxes, Noncurrent Other long-term liabilities Other Long-term Debt, Noncurrent Total liabilities Commitments and contingencies (Note 12) 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; 36,446 and 36,107 issued and outstanding as of December 31, 2018 and 2017, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive loss Total stockholders’ equity Total liabilities and stockholders’ equity Liabilities and Equity Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Contract with Customer, Asset and Liability Contract with Customer, Asset and Liability [Table Text Block] Schedule of Commissions Expense Schedule of Commissions Expense [Table Text Block] Schedule of Commissions Expense [Table Text Block] Schedule of Recognized Identified Assets Acquired and Liabilities Assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Business Acquisition, Pro Forma Information Business Acquisition, Pro Forma Information [Table Text Block] STOCKHOLDERS' EQUITY Stockholders' Equity Note Disclosure [Text Block] INTANGIBLE ASSETS Intangible Assets Disclosure [Text Block] 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 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 Income tax expense REVENUE FROM CONTRACTS WITH CUSTOMERS Revenue from Contract with Customer [Text Block] SEGMENT REPORTING Segment Reporting Disclosure [Text Block] Performance Shares [Member] Performance Shares [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] CoStar Group, Inc. 2007 Stock Incentive Plan [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member] Employee stock incentive plan name. CoStar Group, Inc. 2016 Stock Incentive Plan [Member] CoStar Group, Inc. 2016 Stock Incentive Plan [Member] CoStar Group, Inc. 2016 Stock Incentive Plan [Member] 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 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 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 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 GOODWILL Goodwill 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 Deferred Other Tax Expense (Benefit) Disaggregation of Revenue [Abstract] Restatement [Axis] Restatement [Axis] Restatement [Domain] Restatement [Domain] Restatement Adjustment [Member] Restatement Adjustment [Member] Commissions Sales Commissions and Fees Change in Contract with Customer, Liability [Roll Forward] Change in Contract with Customer, Liability [Roll Forward] Change in Contract with Customer, Liability [Roll Forward] Beginning balance Contract with Customer, Liability Cumulative effect of adoption of ASC 606 Contract with Customer, Asset, Adoption of New Accounting Principle Contract with Customer, Asset, Adoption of New Accounting Principle Revenue recognized in the current period from the amounts in the beginning balance Contract with Customer, Liability, Revenue Recognized New deferrals, net of amounts recognized in the current period Contract with Customer, Liability, New Deferrals, Net of Amounts Recognized in Current Period Contract with Customer, Liability, New Deferrals, Net of Amounts Recognized in Current Period Effects of foreign currency Contract with Customer, Asset, Effects of Currency Translation Contract with Customer, Asset, Effects of Currency Translation Ending balance Statement of Comprehensive Income [Abstract] Other comprehensive (loss) 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), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Total other comprehensive (loss) income Total comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Weighted Average [Member] Weighted Average [Member] Measurement Input Type [Axis] Measurement Input Type [Axis] Measurement Input Type [Domain] Measurement Input Type [Domain] Measurement Input, Discount Rate [Member] Measurement Input, Discount Rate [Member] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Asset Class [Domain] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [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] Financial Instrument [Axis] Money Market Funds [Member] Money Market Funds [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 Debt securities, measurement input Debt Securities, Available-for-sale, Measurement Input Temporary impairment of the auction rates security investments COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] Balance at beginning of year SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Charged to expense SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Cost and Expense Write-offs, net of recoveries SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction Balance at end of year Retirement Benefits [Abstract] Defined Contribution Plan [Table] Defined Contribution Plan [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] 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] Pension Plan [Member] 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] Maximum percentage of employee total compensation matched by employer (in hundredths) Defined Contribution Plan, Employer Matching Contribution, Percent of Match Maximum annual employee contribution Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 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 (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) LONG-TERM DEBT Long-term Debt [Text Block] Contract with customer, asset, gross Contract with Customer, Asset, Gross Property, Plant and Equipment, Type [Axis] Furniture and Office Equipment [Member] Vehicles [Member] Vehicles [Member] Property and equipment, estimated useful life (in years) Property, Plant and Equipment, Useful Life Income Statement [Abstract] Cost of Goods and Services Sold Cost of Goods and Services Sold Operating expenses: Operating Expenses [Abstract] Selling and marketing (excluding customer base amortization) 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 Income from operations Weighted average outstanding shares — basic (in shares) Weighted average outstanding shares — diluted (in shares) Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Acquired Building Photography [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 Performance-based RSAs - with Market Condition [Member] Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Risk-free interest rate 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 Income Tax Contingency [Table] Income Tax Contingency [Table] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] 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 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 reversal Income Tax Penalties and Interest Expense Reversal Income Tax Penalties and Interest Expense Reversal Interest and penalties accrued on income taxes Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued 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] 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 new accounting pronouncements and changes in accounting principles Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block] 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) 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 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 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Share Based Compensation Exercisable Range [Abstract] Share Based Compensation Exercisable Range [Abstract] Share Based Compensation Exercisable Range [Abstract] Outstanding (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 (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 (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 (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 (dollars per share) Exercisable at end of period 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. Schedule II - Valuation and Qualifying Accounts SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Cash paid for acquisitions, net of cash acquired Initial payment for acquisiton Initial Payments to Acquire Businesses, Net of Cash Acquired Initial Payments to Acquire Businesses, Net of Cash Acquired Goodwill acquired Intangible assets acquired Finite-lived Intangible Assets Acquired Previously Reported [Member] Previously Reported [Member] 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 Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Deferred tax liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Contingent sales tax liability State uncertain income tax position liability Other assets and liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets And Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets And Liabilities Fair value of identifiable net assets acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net PROPERTY AND EQUIPMENT Property, Plant and Equipment Disclosure [Text Block] Performance Based RSAs Without Market Condition [Member] Performance Based RSAs Without Market Condition [Member] Performance Based RSAs Without Market Condition [Member]