Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Development costs	$ 28,498	$ 21,216
Employee-related expenses	15,585	19,638
Professional services	14,299	10,903
Other accrued expenses	530	237
Total accrued expenses	$ 58,912	$ 51,994