Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Investments:
Total investments	$ 31,322,000	$ 126,605,000
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	209,830,000	18,429,000
Total cash equivalents	209,830,000	18,429,000
Investments:
Commercial paper		14,214,000
Corporate bonds		32,485,000
Asset-backed securities	6,112,000	8,100,000
U.S. government securities	25,210,000	78,082,000
Total investments	31,322,000	132,881,000
Total cash equivalents and investments	241,152,000	151,310,000
Level 1 | Recurring basis
Cash equivalents:
Money market funds	209,830,000	18,429,000
Total cash equivalents	209,830,000	18,429,000
Investments:
Commercial paper		0
Corporate bonds		0
Asset-backed securities	0	0
U.S. government securities	25,210,000	78,082,000
Total investments	25,210,000	78,082,000
Total cash equivalents and investments	235,040,000	96,511,000
Level 2 | Recurring basis
Cash equivalents:
Money market funds	0	0
Total cash equivalents	0	0
Investments:
Commercial paper		14,214,000
Corporate bonds		32,485,000
Asset-backed securities	6,112,000	8,100,000
U.S. government securities	0	0
Total investments	6,112,000	54,799,000
Total cash equivalents and investments	6,112,000	54,799,000
Level 3 | Recurring basis
Cash equivalents:
Money market funds	0	0
Total cash equivalents	0	0
Investments:
Commercial paper		0
Corporate bonds		0
Asset-backed securities	0	0
U.S. government securities	0	0
Total investments	0	0
Total cash equivalents and investments	$ 0	$ 0