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]
Research and Development	$ 1,023	$ 1,605
Payroll and other employee-related expenses	9,309	3,393
Professional services fees	2,591	927
Interest expense	1,249	161
Other	211	159
Total accrued expenses and other current liabilities	$ 14,383	$ 6,245