Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Research and development	$ 833	$ 969
Payroll and other employee-related expenses	5,242	9,309
Professional services fees	3,753	2,591
Interest expense	1,219	1,249
Accrual for employee stock purchase plan	132	141
Other	461	124
Total accrued expenses and other current liabilities	$ 11,640	$ 14,383