Fair Value Measurements - Schedule of Fair Value and Amortized Cost of Investments in Marketable Securities by Major Security Type (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 884,914	$ 735,042
Unrealized Gain	56	57
Unrealized Loss	(58)	(13)
Fair Value	884,912	735,086
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	362,762	146,468
Fair Value	362,762	146,468
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	206,422	301,075
Unrealized Gain	40	38
Unrealized Loss	(2)	(1)
Fair Value	206,460	301,112
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	5,000	24,997
Unrealized Gain	2	4
Fair Value	5,002	25,001
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	310,730	262,502
Unrealized Gain	14	15
Unrealized Loss	(56)	(12)
Fair Value	$ 310,688	$ 262,505