Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued external research and development expenses	$ 1,770	$ 627
Accrued payroll and related expenses	1,369	1,018
Accrued professional fees	878	1,062
Accrued other	304	221
Accrued expenses and other current liabilities	$ 4,321	$ 2,928