Fair Value Measurements - Schedule of Fair Value and Amortized Cost of Investments in Marketable Securities by Major Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 735,042	$ 188,206
Unrealized Gain	57	67
Unrealized Loss	(13)	(3)
Fair Value	735,086	188,270
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	146,468	45,498
Fair Value	146,468	45,498
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	301,075	74,801
Unrealized Gain	38	12
Unrealized Loss	(1)	(1)
Fair Value	301,112	74,812
U.S. Government Agency Obligations
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	262,502	67,907
Unrealized Gain	15	55
Unrealized Loss	(12)	(2)
Fair Value	$ 262,505	$ 67,960