Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 18,792	$ 14,541
Employee-related expenses	2,145	5,948
Professional services	2,715	1,751
Other accrued expenses		112
Total accrued expenses	$ 23,652	$ 22,352