Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Estimated useful life of assets	10 years
Accrued research and development services current		$ 21,962	$ 29,033
Accrued compensation and benefits current		6,200	9,644
Operating Lease, Liability, Current		553	553
Other Accrued Liabilities, Current		976	2,921
Accrued Liabilities		$ 29,691	$ 42,151
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets		3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of assets		40 years