Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Payroll and employee-related costs	$ 4,696	$ 5,550
Research costs	4,921	2,285
Licensing fees	216	609
Professional fees	1,487	2,176
Intellectual property costs	1,352	500
Other	30	241
Total	$ 12,702	$ 11,361