Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued compensation	$ 7,006	$ 18,982
Accrued research and development costs	18,661	21,777
Accrued professional fees	9,238	8,335
Accrued other	4,726	4,048
Total accrued expenses	$ 39,631	$ 53,142