Fair Value Measurements and Fair Value of Financial Instruments - Unrealized Gain (Loss) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	$ 516,712	$ 431,808
Unrealized Gains	10	57
Unrealized Losses	(44)	(2)
Fair Value	516,678	431,863
Money market funds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	369,580	333,585
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	369,580	333,585
U.S. Treasury securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	147,132	98,223
Unrealized Gains	10	57
Unrealized Losses	(44)	(2)
Fair Value	$ 147,098	$ 98,278