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Streamline Period Applies [Member] Streamline Period Applies [Member] Goodwill and Intangible Assets Goodwill and Intangible Assets Disclosure [Text Block] Summary of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Trading Symbol Trading Symbol Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Capitalized stock-based compensation Share-based Payment Arrangement, Amount Capitalized Stock-based compensation Share-based Payment Arrangement, Noncash Expense Taxes paid related to net share settlement Payment, Tax Withholding, Share-based Payment Arrangement Commitment period Long-term Purchase Commitment, Period Merger and Restructuring Cost Activity Restructuring and Related Costs [Table Text Block] Acceleration of single trigger equity awards, converted Business Combination, Consideration Transferred, Conversion of Stock, Amount Converted Business Combination, Consideration Transferred, Conversion of Stock, Amount Converted Stock options exchanged pre-acquisition Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Stock Options Exchanged Pre-Acquisition Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Stock Options Exchanged Pre-Acquisition Loan Agreement Sublimit Loan Agreement Sublimit [Member] Loan Agreement Sublimit Debt outstanding amount Long-term Debt Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Axis] Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Lease Obligations Lessee, Operating Leases [Text Block] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] City Area Code City Area Code Customer relationships Customer Relationships [Member] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] 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Credit Loss, Recovery Sales and marketing Selling and Marketing Expense Common stock, shares, issued and outstanding (in shares) Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Entity Shares Converted Per Acquiree Share Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Entity Shares Converted Per Acquiree Share Lease liabilities - non-current portion Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Lease, Noncurrent Liabilities Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Lease, Noncurrent Liabilities Cash paid for restructuring costs Payments For Restructuring And Merger Related Costs Payments For Restructuring And Merger Related Costs Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Stock-based compensation expense Share-based Payment Arrangement, Expense Balance Sheet Location [Axis] Balance Sheet Location [Axis] Issuance of common stock related to employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Common stock, $0.00001 par value; 500,000 shares authorized at September 30, 2020 and December 31, 2019; 110,712 and 53,888 shares issued and outstanding at September 30, 2020 and December 31, 2019, respectively Common Stock, Value, Issued Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Deferred tax assets, non-current Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets 2022 Finite-Lived Intangible Asset, Expected Amortization, Year Two Total assets acquired Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Assets Acquired Including Goodwill Business Combination, Recognized Identifiable Assets 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Combination, Consideration Transferred Concentration Risk Type [Domain] Concentration Risk Type [Domain] TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities and Equity Issued in merger (in shares) Business Acquisition, Equity Interest Issued or Issuable, Number of Shares Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Income Taxes Income Tax Disclosure [Text Block] Accrued employee-related payables Employee-related Liabilities, Current Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Unvested restricted stock units Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Accounts payable and accrued expenses Total Accounts Payable and Accrued Liabilities, Current Maximum employee subscription rate Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate Total purchase price Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Total liabilities assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities General and administrative General and Administrative Expense Financial Instrument [Axis] Financial Instrument [Axis] Total shares excluded from net loss per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Short-term lease expense Short-term Lease, Cost Restricted stock units exchanged pre-acquisition Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Restricted Stock Units Exchanged Pre-Acquisition Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Restricted Stock Units Exchanged Pre-Acquisition (Gain) loss on disposal of property and equipment Gain (Loss) on Disposition of Property Plant Equipment Estimated Useful Life Finite-Lived Intangible Asset, Useful Life Equity Component [Domain] Equity Component [Domain] Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two Thereafter Lessee, Operating Lease, Liability, To Be Paid, Due After Year Four Lessee, Operating Lease, Liability, To Be Paid, Due After Year Four Property and equipment, net Property, Plant and Equipment, Net 2023 Finite-Lived Intangible Asset, Expected Amortization, Year Three Debt Instrument Covenant Compliance Streamline Period [Axis] Debt Instrument Covenant Compliance Streamline Period [Axis] Debt Instrument Covenant Compliance Streamline Period [Axis] Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Stock-Based Compensation Share-based Payment Arrangement [Text Block] Change in restricted cash Increase (Decrease) in Restricted Cash Net loss Net loss Net loss Net loss Net Income (Loss) Attributable to Parent Entity Address, State or Province Entity Address, State or Province Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Other current liabilities Other Liabilities, Current Remaining 2020 Lessee, Operating Lease, Liability, to be Paid, Remainder of Fiscal Year London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] Leases [Abstract] Leases [Abstract] Percent of revenue (less than) Percent Of Revenue, Gross Basis Percent Of Revenue, Gross Basis TOTAL CURRENT ASSETS Assets, Current Net loss per share: Earnings Per Share [Abstract] Personnel related (severance and one-time termination benefit costs) Restructuring Charges Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Unrealized foreign currency gains, net Foreign Currency Transaction Gain (Loss), Unrealized Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] STOCKHOLDERS' EQUITY Stockholders' Equity Attributable to Parent [Abstract] Developed technology Developed Technology Rights [Member] Entity Interactive Data Current Entity Interactive Data Current Shares Under Option Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Customer Two Customer Two [Member] Customer Two Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Restructuring costs, Merger assumed loss contracts Restructuring Charges, Merger Assumed Loss Contracts Restructuring Charges, Merger Assumed Loss Contracts SUPPLEMENTAL DISCLOSURES OF OTHER CASH FLOW INFORMATION: Supplemental Cash Flow Information [Abstract] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Issuance of common stock related to employee stock purchase plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Financial Standby Letter of Credit Financial Standby Letter of Credit [Member] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs for all Plans Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] Common Stock  Common Stock [Member] Unrecognized employee stock-based compensation, weighted average period Employee Service Share-Based Compensation, Non-Vested Restricted Stock Units, Compensation Cost Not Yet Recognized, Period for Recognition Employee Service Share-Based Compensation, Non-Vested Restricted Stock Units, Compensation Cost Not Yet Recognized, Period for Recognition Available borrowing capacity Line of Credit Facility, Remaining Borrowing Capacity Total identifiable intangible assets, gross Finite-Lived Intangible Assets, Gross Money Market Funds Money Market Funds [Member] Provision for doubtful accounts Increase (decrease) in provision for expected credit losses Accounts Receivable, Credit Loss Expense (Reversal) Exercise of common stock options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Business Combinations Business Combination Disclosure [Text Block] Total other income, net Nonoperating Income (Expense) Schedule of Maturity of Lease Liabilities Lessee, Operating Lease, Liability, Maturity [Table Text Block] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Stock Option Share-based Payment Arrangement, Option [Member] Basis of Presentation and Summary of Significant Accounting Policies Basis of Accounting, Policy [Policy Text Block] Weighted-average common shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Variable interest rate Debt Instrument, Basis Spread on Variable Rate Expenses: Costs and Expenses [Abstract] Risk-free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Subsequent Events [Abstract] Subsequent Events [Abstract] Provision for doubtful accounts Accounts Receivable, Bad Debt Expense Accounts Receivable, Bad Debt Expense Fair value of stock-based awards exchanged Business Combination, Consideration Transferred, Equity Interests Issuable Fair Value Business Combination, Consideration Transferred, Equity Interests Issuable Fair Value Plan Name [Domain] Plan Name [Domain] Revenues Revenue from Contract with Customer [Text Block] Cash paid for interest Interest Paid, Excluding Capitalized Interest, Operating Activities Options to purchase common stock Equity Option [Member] Operating lease right-of-use assets obtained in exchange for new operating lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Additional paid-in capital Additional Paid in Capital 2021 Finite-Lived Intangible Asset, Expected Amortization, Year One Foreign exchange (gain) loss, net Foreign Currency Transaction Gain (Loss), before Tax Provision (benefit) for income taxes Income Tax Expense (Benefit) Schedule of Nonvested Restricted Stock Units Activity Schedule of Nonvested Restricted Stock Units Activity [Table Text Block] CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Unaudited Pro Forma Information Business Acquisition, Pro Forma Information [Table Text Block] Options assumed in merger (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Assumed In Merger, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Assumed In Merger, Weighted Average Exercise Price Vesting After One Year of Service Share-based Payment Arrangement, Tranche One [Member] Shares of Magnite common stock Business Combination, Consideration Transferred, Equity Interests Issued and Issuable Intrinsic values of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Post merger revenue Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual Common stock, acceleration (in shares) Business Acquisition, Equity Interest Issued Or Issuable, Number Of Shares, Acceleration Business Acquisition, Equity Interest Issued Or Issuable, Number Of Shares, Acceleration Net operating loss, federal Business Acquisition, Net Operating Loss, Federal Business Acquisition, Net Operating Loss, Federal Non-compete agreements Noncompete Agreements [Member] Prime Rate Prime Rate [Member] Exercise of common stock options Stock Issued During Period, Value, Stock Options Exercised Net operating loss, state Business Acquisition, Net Operating Loss, State Business Acquisition, Net Operating Loss, State Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Pro Forma Revenue Business Acquisition, Pro Forma Revenue Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Award Type [Axis] Award Type [Axis] Weighted-Average Grant Date Fair Value Weighted-Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Non-cash lease expense Non-Cash Lease Expense Non-Cash Lease Expense Prepaid Expenses and Other Current Assets Prepaid Expenses and Other Current Assets [Member] Total accumulated amortization—intangible assets Finite-Lived Intangible Assets, Accumulated Amortization Maximum Maximum [Member] Offering period Share-based Compensation Arrangement by Share-based Payment Award, Offering Period Share-based Compensation Arrangement by Share-based Payment Award, Offering Period Fair value of restricted stock vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Security Exchange Name Security Exchange Name Other assets, non-current Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Intrinsic value of nonvested unit Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Nonvested Restricted cash Restricted Cash Accumulated Deficit Retained Earnings [Member] Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Grant date fair value of options granted ad assumed (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants And Assumptions In Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants And Assumptions In Period, Weighted Average Grant Date Fair Value 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] Net cash provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Weighted- Average Contractual Life Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Accounts receivable, allowance for credit loss Allowance for doubtful accounts, beginning balance Allowance for doubtful accounts, ending balance Accounts Receivable, Allowance for Credit Loss Award Type [Domain] Award Type [Domain] Document Transition Report Document Transition Report Exercisable (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Unused capacity fee, percentage Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Accretion of available-for-sale securities Accretion Of Available-For-Sale Securities Accretion Of Available-For-Sale Securities Customer One Customer One [Member] Customer One Net Income (Loss) Per Share Earnings Per Share [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Current liabilities: Liabilities, Current [Abstract] Recurring Fair Value, Recurring [Member] Mobile Mobile [Member] Mobile [Member] Shares withheld related to net share settlement (in shares) Share-based Payment Arrangement, Shares Withheld for Tax Withholding Obligation Expired (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price Recently Adopted and Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Basic and Diluted EPS: Earnings Per Share, Basic [Abstract]