Fair Value Measurements - Schedule of Fair Value and Amortized Cost of Investments in Marketable Securities by Major Security Type (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 743,464	$ 735,042
Unrealized Gain	62	57
Unrealized Loss	(154)	(13)
Fair Value	743,372	735,086
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	172,431	146,468
Fair Value	172,431	146,468
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	109,972	301,075
Unrealized Gain	9	38
Unrealized Loss	(35)	(1)
Fair Value	109,946	301,112
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		24,997
Unrealized Gain		4
Fair Value		25,001
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	461,061	262,502
Unrealized Gain	53	15
Unrealized Loss	(119)	(12)
Fair Value	$ 460,995	$ 262,505