Entity Central Index Key Cover [Abstract] Document And Entity Information Document and entity information Document And Entity Information [Table] Document And Entity Information [Table] Class of Stock Statement Class Of Stock [Axis] Class of Stock Class Of Stock [Domain] Class A Common Stock Common Class A [Member] Class B Common Stock Common Class B [Member] Document And Entity Information [Line Items] Document And Entity Information [Line Items] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Entity Registrant Name Entity Registrant Name Trading Symbol Trading Symbol Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Entity Shell Company Entity Shell Company Entity Current Reporting Status Entity Current Reporting Status Entity File Number Entity File Number Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address Address Line1 Entity Address, Address Line Two Entity Address Address Line2 Entity Address, City or Town Entity Address City Or Town Entity Address, State or Province Entity Address State Or Province Entity Address, Postal Zip Code Entity Address Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Entity Interactive Data Current Entity Interactive Data Current Document Quarterly Report Document Quarterly Report Document Transition Report Document Transition Report Entity Incorporation, State or Country Code Entity Incorporation State Country Code Title of 12(b) Security Security12b Title Security Exchange Name Security Exchange Name Liabilities convertible preferred stock and stockholders' deficit equity. Accrued liabilities accrued income taxes and other liabilities current. Statement Of Financial Position [Abstract] Statement [Table] Statement [Table] Statement [Line Items] Statement [Line Items] Assets Assets [Abstract] Current assets Assets Current [Abstract] Cash and cash equivalents Cash And Cash Equivalents At Carrying Value Investments Held To Maturity Securities Current Accounts receivable, net of allowance for doubtful accounts of $216 and $479 at September 30, 2019 and December 31, 2018, respectively Accounts Receivable Net Current Prepaid expenses and prepaid income taxes Prepaid Expense Current Deferred contract costs Contract With Customer Asset Net Current Other current assets Other Assets Current Restricted cash Restricted Cash Current Total current assets Assets Current Property and equipment, net Property Plant And Equipment Net Intangible assets Finite Lived Intangible Assets Net Goodwill Goodwill Operating lease right-of-use assets Operating Lease Right Of Use Asset Restricted cash Restricted Cash Noncurrent Deferred tax assets Deferred Tax Assets Net Noncurrent Deferred contract costs, net of current portion Contract With Customer Asset Net Noncurrent Other long–term assets Other Assets Noncurrent Total assets Assets Liabilities and stockholders’ equity Liabilities Convertible Preferred Stock And Stockholders Deficit Equity [Abstract] Current liabilities Liabilities Current [Abstract] Accounts payable Accounts Payable Current Accrued expenses, accrued income taxes and other current liabilities Accrued Liabilities Accrued Income Taxes And Other Liabilities Current Deferred revenue Contract With Customer Liability Current Operating lease liabilities Operating Lease Liability Current Total current liabilities Liabilities Current Operating lease liabilities Operating Lease Liability Noncurrent Deferred tax liabilities Deferred Tax Liabilities Noncurrent Other non–current liabilities Other Liabilities Noncurrent Total liabilities Liabilities Commitments and contingencies (Note 8) Commitments And Contingencies Stockholders’ equity: Stockholders Equity [Abstract] Common stock Common Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Retained earnings Retained Earnings Accumulated Deficit Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income Loss Net Of Tax Total stockholders’ equity Stockholders Equity Total liabilities and stockholders’ equity Liabilities And Stockholders Equity Allowance for doubtful accounts Allowance For Doubtful Accounts Receivable Current Common stock, par value Common Stock Par Or Stated Value Per Share Common stock, shares authorized Common Stock Shares Authorized Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Income Statement [Abstract] Revenue Revenue From Contract With Customer Excluding Assessed Tax Cost of revenue Cost Of Goods And Service Excluding Depreciation Depletion And Amortization Gross profit Gross Profit Operating expenses: Operating Expenses [Abstract] Sales and marketing Selling And Marketing Expense Product, technology, and development Research And Development Expense General and administrative General And Administrative Expense Depreciation and amortization Depreciation And Amortization Total operating expenses Operating Expenses Income from operations Operating Income Loss Other income, net: Nonoperating Income Expense [Abstract] Interest income Investment Income Interest Other income (expense), net Other Nonoperating Income Expense Total other income, net Nonoperating Income Expense Income before income taxes Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest Provision for (benefit from) income taxes Income Tax Expense Benefit Net income Net Income Loss Net income per share attributable to common stockholders: (Note 10) Earnings Per Share Basic And Diluted [Abstract] Basic Earnings Per Share Basic Diluted Earnings Per Share Diluted Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders: Weighted Average Number Of Shares Outstanding [Abstract] Basic Weighted Average Basic Shares Outstanding Pro Forma Diluted Pro Forma Weighted Average Shares Outstanding Diluted Depreciation and amortization expense Cost Of Goods And Services Sold Depreciation And Amortization Statement Of Income And Comprehensive Income [Abstract] Net income Other comprehensive loss: Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract] Foreign currency translation adjustment Other Comprehensive Income Loss Foreign Currency Transaction And Translation Adjustment Net Of Tax Comprehensive income Comprehensive Income Net Of Tax Convertible stock shares converted. Convertible stock converted value. Statement Of Stockholders Equity [Abstract] Retained Earnings (Accumulated Deficit) Statement Equity Components [Axis] Equity Component Equity Component [Domain] Common Stock Common Stock [Member] Additional Paid-in Capital Additional Paid In Capital [Member] Accumulated Other Comprehensive (Loss) Income Accumulated Other Comprehensive Income [Member] Retained Earnings (Accumulated Deficit) Retained Earnings [Member] Beginning balance Beginning balance, Shares Stock–based compensation expense Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value Issuance of common stock upon exercise of stock options Stock Issued During Period Value Stock Options Exercised Issuance of common stock upon exercise of stock options, Shares Stock Issued During Period Shares Stock Options Exercised Issuance of common stock upon vesting of restricted stock units Stock Issued During Period Value Restricted Stock Award Net Of Forfeitures Issuance of common stock upon vesting of restricted stock units, Shares Stock Issued During Period Shares Restricted Stock Award Net Of Forfeitures Payment of withholding taxes and option costs on net share settlement of restricted stock units and stock options Adjustments Related To Tax Withholding For Share Based Compensation Payment of withholding taxes and option costs on net share settlement of restricted stock units and stock options, Shares Shares Paid For Tax Withholding For Share Based Compensation Cumulative adjustment from adoption of revenue recognition model Cumulative Effect On Retained Earnings Net Of Tax1 Conversion of common stock Stock Issued During Period Value Conversion Of Convertible Securities Net Of Adjustments Conversion of common stock, shares issued Stock Issued During Period Shares Conversion Of Convertible Securities Conversion of common stock, shares converted Convertible Stock Shares Converted Conversion of common stock, value Convertible Stock Converted Value Ending balance Ending balance, Shares Increase (decrease) in accrued liabilities accrued income taxes and other liabilities current. Increase decrease in lease obligations. Payments of deferred initial public offering costs. Payment of withholding taxes and option costs on net share settlement of restricted stock units and stock options. Statement Of Cash Flows [Abstract] Operating Activities Net Cash Provided By Used In Operating Activities [Abstract] Net income Profit Loss 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 Currency gain on foreign denominated transactions Foreign Currency Transaction Gain Loss Before Tax Deferred taxes Deferred Income Tax Expense Benefit Provision for doubtful accounts Provision For Doubtful Accounts Stock-based compensation expense Share Based Compensation Amortization of deferred contract costs Capitalized Contract Cost Amortization Changes in operating assets and liabilities: Increase Decrease In Operating Capital [Abstract] Accounts receivable Increase Decrease In Accounts Receivable Prepaid expenses, prepaid income taxes, and other assets Increase Decrease In Prepaid Deferred Expense And Other Assets Deferred contract costs Increase Decrease In Contract With Customer Asset Accounts payable Increase Decrease In Accounts Payable Accrued expenses, accrued income taxes, and other current liabilities Increase Decrease In Accrued Liabilities Accrued Income Taxes And Other Liabilities Current Deferred revenue Increase Decrease In Contract With Customer Liability Lease obligations Increase Decrease In Lease Obligations Other non-current liabilities Increase Decrease In Other Noncurrent Liabilities 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] Purchases of property and equipment Payments To Acquire Property Plant And Equipment Capitalization of website development costs Payments To Develop Software Cash paid for acquisition Payments To Acquire Businesses Gross Investments in certificates of deposit Payments To Acquire Restricted Certificates Of Deposit Maturities of certificates of deposit Proceeds From Sale Maturity And Collections Of Investments 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] Proceeds from exercise of stock options Proceeds From Stock Options Exercised Financing cash flows from finance leases Finance Lease Principal Payments Payment of initial public offering costs Payments Of Deferred Initial Public Offering Costs Payment of withholding taxes and option costs on net share settlement of restricted stock units and stock options Payment Of Withholding Taxes And Option Costs On Net Share Settlement Of Restricted Stock Units And Stock Options Net cash used in financing activities Net Cash Provided By Used In Financing Activities Impact of foreign currency on cash, cash equivalents, and restricted cash Effect Of Exchange Rate On Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Including Disposal Group And Discontinued Operations Net decrease in cash, cash equivalents, and restricted cash Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect Cash, cash equivalents, and restricted cash at beginning of period Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Including Disposal Group And Discontinued Operations Cash, cash equivalents, and restricted cash at end of period Supplemental disclosure of cash flow information: Supplemental Cash Flow Information [Abstract] Cash paid for income taxes Income Taxes Paid Unpaid purchases of property and equipment and internal-use software Capital Expenditures Incurred But Not Yet Paid Capitalized stock-based compensation expense in website development and internal-use software costs Employee Service Share Based Compensation Allocation Of Recognized Period Costs Capitalized Amount Cash paid for operating lease liabilities Operating Lease Payments Accounting Policies [Abstract] Organization and Business Description Organization Consolidation Basis Of Presentation Business Description And Accounting Policies [Text Block] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Business Combinations [Abstract] Acquisitions Business Combination Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments Financial Instruments Disclosure [Text Block] Property Plant And Equipment [Abstract] Property and Equipment, Net Property Plant And Equipment Disclosure [Text Block] Goodwill And Intangible Assets Disclosure [Abstract] Goodwill and Other Intangible Assets Goodwill And Intangible Assets Disclosure [Text Block] Accrued Liabilities Current [Abstract] Accrued Expenses, Accrued Income Taxes and Other Current Liabilities Accounts Payable Accrued Liabilities And Other Liabilities Disclosure Current [Text Block] Commitments And Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments And Contingencies Disclosure [Text Block] Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] Stock-based Compensation Disclosure Of Compensation Related Costs Share Based Payments [Text Block] Earnings Per Share [Abstract] Earnings Per Share Earnings Per Share [Text Block] Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Segment Reporting [Abstract] Segment and Geographic Information Segment Reporting Disclosure [Text Block] Basis of Presentation Basis Of Accounting Policy Policy [Text Block] Principles of Consolidation Consolidation Policy [Text Block] Subsequent Event Considerations Subsequent Events Policy Policy [Text Block] Use of Estimates Use Of Estimates Concentration of Credit Risk Concentration Risk Credit Risk Revenue Recognition Revenue From Contract With Customer Policy [Text Block] Recent Accounting Pronouncements Adopted New Accounting Pronouncements Policy Policy [Text Block] Lease Accounting Lessee Leases Policy [Text Block] Stock-Based Compensation Share Based Compensation Option And Incentive Plans Policy Internal-Use Software Internal Use Software Policy Recent accounting pronouncements not yet adopted. Recent Accounting Pronouncements Not Yet Adopted Recent Accounting Pronouncements Not Yet Adopted Policy [Text Block] Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments Fair Value Of Financial Instruments Policy Goodwill and Other Intangible Assets Goodwill And Intangible Assets Policy [Text Block] Earnings Per Share Earnings Per Share Policy [Text Block] Summary of Revenue from Contracts with Customers by Revenue Disaggregation Of Revenue Table [Text Block] Summary of Future Minimum Lease Payments Lessee Operating Lease Liability Maturity Table [Text Block] Summary of Preliminary Purchase Price Allocation Schedule Of Recognized Identified Assets Acquired And Liabilities Assumed Table [Text Block] Schedule of Fair Value Levels, Assets Measured at Fair Value on Recurring Basis Fair Value Assets Measured On Recurring Basis [Text Block] Schedule of Cash, Cash Equivalents, and Investments Schedule Of Cash Cash Equivalents And Short Term Investments Table [Text Block] Schedule of Property and Equipment Property Plant And Equipment [Text Block] Summary of Changes in Carrying Value of Goodwill Schedule Of Goodwill [Text Block] Summary of Other Intangible Assets Schedule Of Finite Lived Intangible Assets Table [Text Block] Summary of Estimated Amortization Expense of Intangible Assets Scheduleof Finite Lived Intangible Assets Future Amortization Expense Table [Text Block] Schedule of Accrued Expenses, Accrued Income Taxes and Other Current Liabilities Schedule Of Accrued Liabilities Table [Text Block] Summary of Stock-based Compensation Expense by Award Type Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation Of Share Based Compensation Costs By Plan Table [Text Block] Summary of Allocation of Stock-based Compensation Expense Schedule Of Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Text Block] Schedule of Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Income Per Share Schedule Of Earnings Per Share Basic And Diluted Table [Text Block] Schedule of Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Weighted-average Shares Outstanding Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Text Block] Summary of Operations by Segment and Geographical Area Schedule Of Segment Reporting Information By Segment [Text Block] Organization Consolidation And Presentation Of Financial Statements [Abstract] State of incorporation Date of incorporation Entity Incorporation Date Of Incorporation Summary of significant accounting policies. Summary of significant accounting policies. Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Concentration Risk Benchmark Concentration Risk By Benchmark [Axis] Concentration Risk Benchmark Concentration Risk Benchmark [Domain] Sales Revenue, Net Sales Revenue Net [Member] Concentration Risk Type Concentration Risk By Type [Axis] Concentration Risk Type Concentration Risk Type [Domain] Concentration of Credit Risk Customer Concentration Risk [Member] Customer Major Customers [Axis] Customer Name Of Major Customer [Domain] Customer one. Customer One Customer One [Member] Customer two. Customer Two Customer Two [Member] Net Accounts Receivable Accounts Receivable [Member] Advertising customers. Advertising Customers Advertising Customers [Member] Geographical Statement Geographical [Axis] Geographical Segment Geographical [Domain] Cambridge, Massachusetts at 55 Cambridge Parkway. Cambridge, Massachusetts at 55 Cambridge Parkway Cambridge Massachusetts Fifty Five Cambridge Parkway [Member] Needham, Massachusetts MASSACHUSETTS Cambridge, Massachusetts at 121 First Street. Cambridge, Massachusetts at 121 First St. Cambridge Massachusetts One Two One First Street [Member] Dublin, Ireland IRELAND Cambridge, Massachusetts at 2 Canal Park. Cambridge, Massachusetts at 2 Canal Park Cambridge Massachusetts Two Canal Park [Member] Adjustments for New Accounting Pronouncements Adjustments For New Accounting Pronouncements [Axis] Type of Adoption Type Of Adoption [Member] ASC 842 Accounting Standards Update201602 [Member] Summary Of Significant Accounting Policies [Line Items] Summary Of Significant Accounting Policies [Line Items] Description of significant off-balance sheet risk Concentration Risk Credit Risk Financial Instruments Off Balance Sheet Risk Number of major customers. Concentration of credit risk, percentage Concentration Risk Percentage1 Number of major customers Number Of Major Customers Unbilled accounts receivable Unbilled Receivables Current Disaggregation Of Revenue [Table] Disaggregation Of Revenue [Table] Product and Service Product Or Service [Axis] Product and Service Products And Services [Domain] Marketplace subscription revenue. Marketplace Subscription Revenue Marketplace Subscription Revenue [Member] Advertising and other revenue. Advertising and Other Revenue Advertising And Other Revenue [Member] Disaggregation Of Revenue [Line Items] Disaggregation Of Revenue [Line Items] Revenue by Revenue Stream [Abstract] Disaggregation Of Revenue [Abstract] Performance obligation unsatisfied Revenue Remaining Performance Obligation Revenue recognized Contract With Customer Liability Revenue Recognized Operating lease of office space Land Subject To Ground Leases Operating lease, option to extend Lessee Operating Lease Option To Extend Lessee, operating lease, renewal term Lessee Operating Lease Renewal Term Lessee, operating lease, existence of option to terminate Lessee Operating Lease Existence Of Option To Terminate Operating lease rent increasing year. Operating lease, rent increasing year Operating Lease Rent Increasing Year Restricted cash Restricted Cash Net lease assets Lease liabilities Operating Lease Liability Lease costs Lease Cost Weighted average remaining lease term Operating Lease Weighted Average Remaining Lease Term1 Weighted average discount rate Operating Lease Weighted Average Discount Rate Percent Operating Lease Commitments Operating Lease Liabilities Payments Due [Abstract] 2019 (excluding the nine months ended September 30, 2019) Lessee Operating Lease Liability Payments Remainder Of Fiscal Year 2020 Lessee Operating Lease Liability Payments Due Year Two 2021 Lessee Operating Lease Liability Payments Due Year Three 2022 Lessee Operating Lease Liability Payments Due Year Four 2023 Lessee Operating Lease Liability Payments Due Year Five Thereafter Lessee Operating Lease Liability Payments Due After Year Five Total lease payments Lessee Operating Lease Liability Payments Due Less imputed interest Lessee Operating Lease Liability Undiscounted Excess Amount Total Lessee operating lease lease not yet commenced option to extend amount. Future minimum lease payments for leases signed but not yet commenced Lessee Operating Lease Lease Not Yet Commenced Option To Extend Amount Software implementation costs. Implementation costs associated with service contracts Software Implementation Costs Schedule Of Business Acquisitions By Acquisition [Table] Schedule Of Business Acquisitions By Acquisition [Table] Business Acquisition Business Acquisition [Axis] Business Acquisition, Acquiree Business Acquisition Acquiree [Domain] PistonHeads Holdco Limited. PistonHeads Holdco Limited Piston Heads Holdco Limited [Member] United Kingdom UNITED KINGDOM Collaborative Arrangement and Arrangement Other than Collaborative Type Of Arrangement [Axis] Collaborative Arrangement and Arrangement Other than Collaborative Arrangements And Nonarrangement Transactions [Member] Put and call option agreement. Put and Call Option Agreement Put And Call Option Agreement [Member] Escrow agreement. Escrow Agreement Escrow Agreement [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Payments to acquire business Acquisition-related costs incurred Business Combination Acquisition Related Costs Business acquisition completed date Business Acquisition Effective Date Of Acquisition1 Business combination recognized identifiable assets acquired goodwill and liabilities assumed after adjustments. Business combination recognized identifiable assets acquired and liabilities assumed intangibles after adjustment. Goodwill after adjustment. Business combination recognized identifiable assets acquired and liabilities assumed deferred tax liabilities noncurrent after adjustments. Business combination recognized identifiable assets acquired goodwill and liabilities assumed net adjustment. Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Intangibles Adjustment Business combination recognized identifiable assets acquired and liabilities assumed deferred tax liabilities noncurrent adjustment. Intangible assets, Estimated fair value Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Intangibles Deferred tax liabilities, Estimated Fair Value Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deferred Tax Liabilities Noncurrent Total purchase price, Estimated fair value Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Net Intangible assets, Adjustment Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Intangibles Adjustment Goodwill, Adjustment Goodwill Purchase Accounting Adjustments Deferred tax liabilities, Adjustment Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deferred Tax Liabilities Noncurrent Adjustment Total purchase price, Adjustment Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Net Adjustment Intangible assets, Adjusted fair value Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Intangibles After Adjustment Goodwill, Adjusted fair value Goodwill After Adjustment Deferred tax liabilities, Adjusted fair value Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deferred Tax Liabilities Noncurrent After Adjustments Total purchase price , Adjusted fair value Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed After Adjustments Finite-Lived Intangible Assets by Major Class Finite Lived Intangible Assets By Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name Finite Lived Intangible Assets Major Class Name [Domain] Brand. Brand Brand [Member] Customer Relationships Customer Relationships [Member] Intangible assets Estimated useful lives Finite Lived Intangible Asset Useful Life Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Measurement Frequency Fair Value By Measurement Frequency [Axis] Measurement Frequency Fair Value Measurement Frequency [Domain] Fair Value, Measurements, Recurring Fair Value Measurements Recurring [Member] Asset Class Fair Value By Asset Class [Axis] Asset Class Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation By Asset Class [Domain] Money Market Funds Money Market Funds [Member] Fair Value Hierarchy and NAV Fair Value By Fair Value Hierarchy Level [Axis] Fair Value Hierarchy and NAV Fair Value Measurements Fair Value Hierarchy [Domain] Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs) Fair Value Inputs Level1 [Member] Significant Other Observable Inputs (Level 2 Inputs) Fair Value Inputs Level2 [Member] Significant Unobservable Inputs (Level 3 Inputs) Fair Value Inputs Level3 [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] Cash equivalents: Cash And Cash Equivalents Fair Value Disclosure Investments: Debt Securities, Held-to-maturity, Type [Extensible List] Debt Securities Held To Maturity Type Extensible List Total Assets Fair Value Disclosure Maturity period of certificates of deposit Maturity Of Time Deposits Maturity of certificates of deposit, description Maturities Of Time Deposits Description Liabilities, fair value Liabilities Fair Value Disclosure Cash and cash equivalents and held to maturity amortized cost. Cash and cash equivalents and held to maturity fair value. Cash and cash equivalents due in 90 days or less, Amortized Cost Certificates of deposit due in one year or less, Amortized Cost Held To Maturity Securities Total cash, cash equivalents, and investments, Amortized Cost Cash And Cash Equivalents And Held To Maturity Amortized Cost Cash and cash equivalents due in 90 days or less, Estimated Fair Value Certificates of deposit due in one year or less, Estimated Fair Value Held To Maturity Securities Fair Value Total cash, cash equivalents, and investments, Estimated Fair Value Cash And Cash Equivalents And Held To Maturity Fair Value Schedule Of Property Plant And Equipment [Table] Schedule Of Property Plant And Equipment [Table] Property, Plant and Equipment, Type Property Plant And Equipment By Type [Axis] Property, Plant and Equipment, Type Property Plant And Equipment Type [Domain] Computer Equipment Computer Equipment [Member] Capitalized Software Software And Software Development Costs [Member] Capitalized Website Development Costs Software Development [Member] Furniture and Fixtures Furniture And Fixtures [Member] Leasehold Improvements Leasehold Improvements [Member] Construction in Progress Construction In Progress [Member] Finance lease right-of-use assets. Finance Lease Right-of-use Assets Finance Lease Right Of Use Assets [Member] Property Plant And Equipment [Line Items] Property Plant And Equipment [Line Items] Property and equipment, gross Property Plant And Equipment Gross Less accumulated depreciation and amortization Accumulated Depreciation Depletion And Amortization Property Plant And Equipment Property and equipment, net Depreciation Depreciation Increase (decrease) in property and equipment Property Plant And Equipment Gross Period Increase Decrease Balance at December 31, 2018 Initial value of PistonHeads acquisition Goodwill Acquired During Period Foreign currency translation adjustment Goodwill Foreign Currency Translation Gain Loss Purchase price adjustment Balance at September 30, 2019 Schedule Of Finite Lived Intangible Assets [Table] Schedule Of Finite Lived Intangible Assets [Table] Finite Lived Intangible Assets [Line Items] Finite Lived Intangible Assets [Line Items] Weighted Average Remaining Useful Life (years) Acquired Finite Lived Intangible Assets Weighted Average Useful Life Gross Carrying Amount Finite Lived Intangible Assets Gross Accumulated Amortization Finite Lived Intangible Assets Accumulated Amortization Net Carrying Amount Schedule Of Goodwill [Table] Schedule Of Goodwill [Table] Goodwill [Line Items] Goodwill [Line Items] Amortization expense of intangible assets Amortization Of Intangible Assets Estimated useful life Remainder of 2019 Finite Lived Intangible Assets Amortization Expense Remainder Of Fiscal Year 2020 Finite Lived Intangible Assets Amortization Expense Year Two 2021 Finite Lived Intangible Assets Amortization Expense Year Three 2022 Finite Lived Intangible Assets Amortization Expense Year Four 2023 Finite Lived Intangible Assets Amortization Expense Year Five 2024 and thereafter Finite Lived Intangible Assets Amortization Expense After Year Five Other accrued expenses, accrued income taxes and other liabilities Accrued bonus Accrued Bonuses Current Other accrued expenses, accrued income taxes and other current liabilities Other Accrued Expenses Accrued Income Taxes And Other Liabilities Total Unpaid property, equipment and internal-use software costs 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] Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items] Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items] Total stock-based compensation expense Allocated Share Based Compensation Expense 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 Award Type [Axis] Award Type Share Based Compensation Arrangements By Share Based Payment Award Award Type And Plan Name [Domain] Stock Options Employee Stock Option [Member] RSUs Restricted Stock Units R S U [Member] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Income Statement Location Income Statement Location [Axis] Income Statement Location Income Statement Location [Domain] Cost of Revenue Cost Of Sales [Member] Sales and Marketing Expense Selling And Marketing Expense [Member] Product technology and development expense. Product, Technology, and Development Expense Product Technology And Development Expense [Member] General and Administrative Expense General And Administrative Expense [Member] Shares paid for tax withholding and option costs for share-based compensation. Capitalized website development and internal-use software costs excluded from stock-based compensation expense Employee tax withholding requirements and option costs due to net share settlement Shares Paid For Tax Withholding And Option Costs For Share Based Compensation Total payments for employees' tax obligations to taxing authorities due to net share settlements Number of votes entitled to stockholders per share. Undistributed earnings ratio used to calculate allocation to class of stock. Schedule Of Earnings Per Share Basic By Common Class [Table] Schedule Of Earnings Per Share Basic By Common Class [Table] Earnings Per Share Basic [Line Items] Earnings Per Share Basic [Line Items] Right to voting Common Stock Voting Rights Number of votes entitled to stockholders per share Number Of Votes Entitled To Stockholders Per Share Conversion of stock, description Conversion Of Stock Description Class of share converted to another class Conversion Of Stock Type Of Stock Converted Conversion of stock Conversion Of Stock Shares Issued1 Undistributed earnings ratio used to calculate allocation to class of stock Undistributed Earnings Ratio Used To Calculate Allocation To Class Of Stock Numerator: Net Income Loss [Abstract] Denominator: Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic Weighted Average Number Of Shares Outstanding Basic Dilutive effect of share equivalents resulting from stock options Incremental Common Shares Attributable To Share Based Payment Arrangements Dilutive effect of share equivalents resulting from unvested restricted stock units Incremental Common Shares Attributable To Equity Unit Purchase Agreements Weighted-average number of shares of common stock used in computing net income per share — diluted Weighted Average Number Of Diluted Shares Outstanding Net income per share attributable to common stockholders: 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 Antidilutive Securities Excluded From Computation Of Earnings Per Share By Antidilutive Securities [Axis] Antidilutive Securities, Name Antidilutive Securities Name [Domain] Restricted Stock Units Outstanding Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items] Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items] Potentially dilutive common stock equivalents excluded from calculation of diluted weighted-average shares outstanding Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount Income tax expense (benefit) Effective income tax rate Effective Income Tax Rate Continuing Operations Schedule Of Segment Reporting Information By Segment [Table] Schedule Of Segment Reporting Information By Segment [Table] Segments Statement Business Segments [Axis] Segments Segment [Domain] International segment. International International Segment [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Number of reportable segments Number Of Reportable Segments United states segment. United States United States Segment [Member] Segment revenue: Segment Reporting Information Revenue [Abstract] Segment income (loss) from operations: Segment Reporting Information Operating Income Loss [Abstract] Total income from operations Assets