Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Fair Value Measurements
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	$ 0	$ 0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value Measurements
Investments in money market funds	58,042,000	108,585,000
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. Treasury securities
Fair Value Measurements
Investments in money market funds	54,233,000
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities
Fair Value Measurements
Investments in money market funds	0
Recurring basis | Significant other observable inputs (Level 2) | Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Significant other observable inputs (Level 2) | U.S. Treasury securities
Fair Value Measurements
Investments in money market funds	0
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	79,275,000	13,875,000
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Fair Value Measurements
Investments in money market funds	10,166,000
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Significant unobservable inputs (Level 3) | U.S. Treasury securities
Fair Value Measurements
Investments in money market funds	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	0	$ 0
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Fair Value Measurements
Investments in money market funds	$ 0