Income Taxes - Reconciliation of Income Tax Computed at Federal Statutory Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax expense (benefit) at statutory tax rate	$ 13,311	$ (2,812)	$ (5,775)
State tax (benefits), net of federal tax	1,526	(239)	(390)
Permanent differences	635	327	129
Federal and state research credits—current year	(3,628)	(484)	(979)
Accrual of uncertain tax positions	0	142	43
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	(9,286)	(5,851)	0
Impact of change in federal and state tax rate on revaluing deferred tax assets	0	3,357	(4)
Change in valuation allowance	(13,374)	5,799	4,687
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount	(786)
Other	803	(110)	(102)
(Benefit) provision for income taxes	$ (10,799)	$ 129	$ (2,391)