Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Development costs	$ 42,736	$ 21,216
Employee-related expenses	17,391	19,638
Professional services	17,109	10,903
Other accrued expenses	891	237
Total accrued expenses	$ 78,127	$ 51,994