Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Development costs	$ 12,840	$ 6,466
Employee-related expenses	3,010	2,718
Professional services	1,225	935
Other accrued expenses	18	29
Total accrued expenses	$ 17,093	$ 10,148