Fair Value Measurements and Fair Value of Financial Instruments - Unrealized Gain (Loss) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	$ 505,518	$ 431,808
Unrealized Gains	12	57
Unrealized Losses	(20)	(2)
Fair Value	505,510	431,863
Money market funds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	330,308	333,585
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	330,308	333,585
U.S. Treasury securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	175,210	98,223
Unrealized Gains	12	57
Unrealized Losses	(20)	(2)
Fair Value	$ 175,202	$ 98,278