Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 23,473	$ 14,541
Employee-related expenses	15,838	5,948
Professional services	3,166	1,751
Other accrued expenses	124	112
Total accrued expenses	$ 42,601	$ 22,352