Fair Value Measurements and Fair Value of Financial Instruments - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets measured at fair value	$ 516,678	$ 431,863
Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	369,580	333,585
U.S. Treasury securities	147,098	98,278
Total assets measured at fair value	516,678
Level 1 | Fair Value, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Money market funds	369,580	333,585
U.S. Treasury securities	0	0
Total assets measured at fair value	369,580	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	147,098	98,278
Total assets measured at fair value	147,098	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