Fair Value Measurements and Fair Value of Financial Instruments - Unrealized Gain (Loss) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	$ 530,019	$ 431,808
Unrealized Gains	28	57
Unrealized Losses	0	(2)
Fair Value	530,047	431,863
Money market funds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	444,668	333,585
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	444,668	333,585
U.S. Treasury securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Amortized Cost Basis	85,351	98,223
Unrealized Gains	28	57
Unrealized Losses	0	(2)
Fair Value	$ 85,379	$ 98,278