Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and payroll-related costs	$ 1,120	$ 899
External research and development services	1,284	723
Professional fees	536	322
Other	351	156
Total accrued expenses and other current liabilities	$ 3,291	$ 2,100