Fair Value of Financial Instruments - Summary of Fair Value of Financial Instrument (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Money market funds	$ 186,308
Total short-term investments	141,191	$ 16,384
Total	327,499	18,779
Commercial paper		2,395
Liabilities:
Preferred stock purchase right liability	41,600	1,478
Commercial Paper
Assets:
Total short-term investments	53,922	10,357
U.S. Treasury Bills
Assets:
Total short-term investments	45,312
Corporate Debt Securities
Assets:
Total short-term investments	41,957	6,027
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Money market funds	186,308
Total	186,308
Significant Other Observable Inputs (Level 2)
Assets:
Total short-term investments	141,191	16,384
Total	141,191	18,779
Commercial paper		2,395
Significant Other Observable Inputs (Level 2) | Commercial Paper
Assets:
Total short-term investments	53,922	10,357
Significant Other Observable Inputs (Level 2) | U.S. Treasury Bills
Assets:
Total short-term investments	45,312
Significant Other Observable Inputs (Level 2) | Corporate Debt Securities
Assets:
Total short-term investments	$ 41,957	6,027
Significant Unobservable Inputs (Level 3)
Liabilities:
Preferred stock purchase right liability		$ 1,478