Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
External research and development expenses	$ 22,979	$ 20,462
Payroll and related expenses	5,331	12,902
Professional fees	3,153	3,810
Other	771	1,542
Total accrued expenses and other current liabilities	$ 32,234	$ 38,716