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 professional fees	$ 265	$ 247
Accrued compensation and related costs	2,393	1,641
Accrued clinical costs	9,177	4,493
Other	158	390
Total accrued expenses	$ 11,993	$ 6,771