Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued compensation	$ 12,897	$ 18,982
Accrued research and development costs	18,697	21,777
Accrued professional fees	4,393	8,335
Accrued other	5,924	4,048
Total accrued expenses	$ 41,911	$ 53,142