Income Taxes - Reconciliation of Income Tax Computed at Federal Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax benefit at statutory tax rate	$ (3,254)	$ (2,812)	$ (5,775)
State tax benefits, net of federal tax benefit	(235)	(239)	(390)
Permanent differences	157	327	129
Federal and state research credits—current year	(722)	(484)	(979)
Accrual of uncertain tax positions	101	142	43
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount		(5,851)
Impact of change in federal and state tax rate on revaluing deferred tax assets	56	3,357	(4)
Change in valuation allowance	756	5,799	4,687
Other	(77)	(110)	(102)
Provision (benefit) for income taxes	$ (3,218)	$ 129	$ (2,391)	$ (3,218)