Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Development costs	$ 21,216	$ 23,473
Employee-related expenses	19,638	15,838
Professional services	10,903	3,166
Other accrued expenses	237	124
Total accrued expenses	$ 51,994	$ 42,601