Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts Payable and Accrued Liabilities [Abstract]
External research and development expenses	$ 20,462	$ 8,761
Payroll and related expenses	12,902	4,139
Professional fees	3,810	747
Other	1,542	62
Total accrued expenses and other current liabilities	$ 38,716	$ 13,709