Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	$ 65,472	$ 61,104
Cash equivalents and marketable securities, Fair Market Value	65,472	61,104
U.S Government Treasury Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	363,248	227,446
Cash equivalents and marketable securities, Unrealized Gains	1,080	149
Cash equivalents and marketable securities, Unrealized Losses	(7)	(8)
Cash equivalents and marketable securities, Fair Market Value	364,321	227,587
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	4,984	43,735
Cash equivalents and marketable securities, Fair Market Value	4,984	43,735
Corporate Bonds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	10,010	10,103
Cash equivalents and marketable securities, Unrealized Gains	21	3
Cash equivalents and marketable securities, Unrealized Losses		(2)
Cash equivalents and marketable securities, Fair Market Value	10,031	10,104
Cash Equivalents and Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	443,714	342,388
Cash equivalents and marketable securities, Unrealized Gains	1,101	152
Cash equivalents and marketable securities, Unrealized Losses	(7)	(10)
Cash equivalents and marketable securities, Fair Market Value	$ 444,808	$ 342,530