Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued external research and development expenses	$ 17,746	$ 4,541
Accrued payroll and related expenses	2,630	2,379
Accrued professional fees	803	917
Accrued other	409	426
Accrued expenses and other current liabilities	$ 21,588	$ 8,263