Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Accrued Expenses and Other Current Liabilities
External research and development expenses	$ 26,076	$ 20,462
Payroll and related expenses	7,849	12,902
Professional fees	2,381	3,810
Revenue-related reserves	509	0
Other	823	1,542
Total accrued expenses and other current liabilities	37,333	38,716
Accrued Expenses And Other Current Liabilities
Schedule of Accrued Expenses and Other Current Liabilities
Revenue-related reserves	$ 204	$ 0