Fair Value Measurements - Schedule of Fair Value and Amortized Cost of Investments in Marketable Securities by Major Security Type (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 462,329	$ 188,206
Unrealized Gain	151	67
Unrealized Loss	(7)	(3)
Fair Value	462,473	188,270
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	273,190	45,498
Fair Value	273,190	45,498
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	120,894	74,801
Unrealized Gain	121	12
Unrealized Loss		(1)
Fair Value	121,015	74,812
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	14,991
Unrealized Loss	(1)
Fair Value	14,990
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	53,254	67,907
Unrealized Gain	30	55
Unrealized Loss	(6)	(2)
Fair Value	$ 53,278	$ 67,960