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, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 785,051	$ 188,206
Unrealized Gain	87	67
Unrealized Loss	(6)	(3)
Fair Value	785,132	188,270
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	181,303	45,498
Fair Value	181,303	45,498
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	373,423	74,801
Unrealized Gain	54	12
Unrealized Loss	(1)	(1)
Fair Value	373,476	74,812
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	34,986
Unrealized Gain	6
Fair Value	34,992
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	195,339	67,907
Unrealized Gain	27	55
Unrealized Loss	(5)	(2)
Fair Value	$ 195,361	$ 67,960