Business Combinations - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	12 Months Ended
	Jul. 01, 2021	Apr. 30, 2021	Feb. 04, 2021	Apr. 01, 2020	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Apr. 29, 2021
Business Acquisition [Line Items]
Percent of company owned by pre-merger holders of company				52.00%
Merger, acquisition, and restructuring costs						$ 38,177	$ 17,552	$ 2,041
Increase (decrease) in valuation allowance						(53,900)	16,400	2,700
Finite-lived intangible assets acquired	$ 23,400
Goodwill					$ 969,873	969,873	158,125	$ 7,370
Non-compete agreements
Business Acquisition [Line Items]
Estimated useful life					2 years
SpringServe | SpotX, Inc.
Business Acquisition [Line Items]
Prior investment							2,100
Fair value of prior investment	9,500	$ 7,500
Telaria
Business Acquisition [Line Items]
Common stock, issued and outstanding (in shares)				1.082
Issued in merger plus acceleration (in shares)				52,098,945
Percent of company owned by holders of acquired company				48.00%
Issued in merger (in shares)				52,008,316
Stock Consideration (Fair Value of Shares of Magnite common stock)				$ 274,604
Stock options exchanged pre-acquisition				10,400
Restricted stock units exchanged pre-acquisition				$ 1,200
Common stock, acceleration (in shares)				90,629
Acceleration of single trigger equity awards, converted				$ 1,168
Stock options exchanged post-acquisition				4,700
Restricted stock units exchanged post-acquisition				12,200
Net operating loss, federal							126,100
Net operating loss, state							87,600
Post merger revenue							$ 60,100
Purchase consideration				287,418
Deferred tax liability				2,267
Goodwill				$ 150,755
SpotX, Inc.
Business Acquisition [Line Items]
Issued in merger (in shares)		12,374,315	14,000,000
Stock Consideration (Fair Value of Shares of Magnite common stock)		$ 495,591
Merger, acquisition, and restructuring costs						27,900
Cash Consideration		$ 640,000	$ 560,000
Cash consideration, elected increase, percent		20.00%
Cash consideration, elected increase, amount		$ 80,000
Shares issued, elected decrease, percent		20.00%
Shares issued, elected decrease, value		$ 80,000
Share Price (in USD per share)		$ 40.05							$ 49.21
Purchase price prior to working capital adjustments		$ 1,100,000
Working capital adjustment		65,152
Purchase consideration		1,200,743
Increase (decrease) in valuation allowance						(56,200)
Finite-lived intangible assets acquired		429,600
Deferred tax liability		103,039
Goodwill		$ 782,831
SpotX, Inc. | Technology
Business Acquisition [Line Items]
Estimated useful life		5 years
Finite-lived intangible assets acquired		$ 280,400
SpotX, Inc. | Non-compete agreements
Business Acquisition [Line Items]
Estimated useful life		1 year
Finite-lived intangible assets acquired		$ 1,500
SpringServe
Business Acquisition [Line Items]
Merger, acquisition, and restructuring costs						0
Cash Consideration	31,136
Purchase price held back to cover possible indemnification claims	1,400
Deferred tax liability	6,154
Goodwill	$ 24,156
SpringServe | Technology
Business Acquisition [Line Items]
Estimated useful life	5 years
Finite-lived intangible assets acquired	$ 15,500
SpringServe | Non-compete agreements
Business Acquisition [Line Items]
Estimated useful life	2 years
Finite-lived intangible assets acquired	$ 500
Nth Party
Business Acquisition [Line Items]
Purchase consideration					$ 9,000
Deferred tax liability					1,300	1,300
Goodwill					$ 4,800	$ 4,800
Nth Party | Technology
Business Acquisition [Line Items]
Estimated useful life					5 years
Finite-lived intangible assets acquired					$ 5,400
Nth Party | Non-compete agreements
Business Acquisition [Line Items]
Finite-lived intangible assets acquired					$ 200