Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurements
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	$ 0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value Measurements
Investments	82,872	$ 58,042
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. Treasury securities
Fair Value Measurements
Investments	8,999	54,233
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate debt securities
Fair Value Measurements
Investments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities
Fair Value Measurements
Investments	0	0
Recurring basis | Significant other observable inputs (Level 2) | Money market funds
Fair Value Measurements
Investments	0	0
Recurring basis | Significant other observable inputs (Level 2) | U.S. Treasury securities
Fair Value Measurements
Investments	0	0
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Fair Value Measurements
Investments	43,606	79,275
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Fair Value Measurements
Investments	9,486	10,166
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds
Fair Value Measurements
Investments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | U.S. Treasury securities
Fair Value Measurements
Investments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Fair Value Measurements
Investments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Fair Value Measurements
Investments	$ 0	$ 0