Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued research and development costs	$ 46,940	$ 21,216
Employee-related expenses	22,011	19,638
Professional services	16,720	10,903
Other	947	237
Total accrued expenses	$ 86,618	$ 51,994