Fair Value Measurements and Fair Value of Financial Instruments - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	$ 505,510	$ 431,863
Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	330,308	333,585
U.S. Treasury securities	175,202	98,278
Total assets measured at fair value	505,510
Level 1 | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	330,308	333,585
U.S. Treasury securities	0	0
Total assets measured at fair value	330,308	333,585
Level 2 | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	0	0
U.S. Treasury securities	175,202	98,278
Total assets measured at fair value	175,202	98,278
Level 3 | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	0	0
U.S. Treasury securities	0	0
Total assets measured at fair value	$ 0	$ 0