Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Development costs	$ 14,541	$ 6,466
Employee-related expenses	5,948	2,718
Professional services	1,751	935
Other accrued expenses	112	29
Total accrued expenses	$ 22,352	$ 10,148