Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 27,326	$ 14,541
Employee-related expenses	6,299	5,948
Professional services	3,179	1,751
Other accrued expenses	3	112
Total accrued expenses	$ 36,807	$ 22,352