Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 19,305	$ 14,541
Employee-related expenses	4,142	5,948
Professional services	2,792	1,751
Other accrued expenses		112
Total accrued expenses	$ 26,239	$ 22,352