Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Development costs	$ 23,932	$ 21,216
Employee-related expenses	8,676	19,638
Professional services	11,746	10,903
Other accrued expenses	3,297	237
Total accrued expenses	$ 47,651	$ 51,994