Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Investments:
Total investments	$ 43,796,000	$ 43,844,000
Fair value of assets transferred from Level 1 to Level 2	0	0
Fair value of assets transferred from Level 2 to Level 1	0	0
Fair value of liabilities transferred from Level 1 to Level 2	0	0
Fair value of liabilities transferred from Level 2 to Level 1	0	0
Recurring basis
Cash equivalents:
Money market funds	170,391,000	179,389,000
Total cash equivalents	170,391,000	179,389,000
Investments:
Commercial paper	13,286,000	13,275,000
Corporate bonds	6,506,000	6,540,000
U.S. government securities	24,004,000	24,029,000
Total investments	43,796,000	43,844,000
Total cash equivalents and investments	214,187,000	223,233,000
Level 1 | Recurring basis
Cash equivalents:
Money market funds	170,391,000	179,389,000
Total cash equivalents	170,391,000	179,389,000
Investments:
Commercial paper	0	0
Corporate bonds	0	0
U.S. government securities	24,004,000	24,029,000
Total investments	24,004,000	24,029,000
Total cash equivalents and investments	194,395,000	203,418,000
Level 2 | Recurring basis
Cash equivalents:
Money market funds	0	0
Total cash equivalents	0	0
Investments:
Commercial paper	13,286,000	13,275,000
Corporate bonds	6,506,000	6,540,000
U.S. government securities	0	0
Total investments	19,792,000	19,815,000
Total cash equivalents and investments	19,792,000	19,815,000
Level 3 | Recurring basis
Cash equivalents:
Money market funds	0	0
Total cash equivalents	0	0
Investments:
Commercial paper	0	0
Corporate bonds	0	0
U.S. government securities	0	0
Total investments	0	0
Total cash equivalents and investments	$ 0	$ 0