Income Taxes - Reconciliation of Income Tax Computed at Federal Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax benefit at statutory tax rate	$ (3,844)	$ (5,775)	$ (3,254)
State tax benefit, net of federal tax benefit	(151)	(390)	(235)
Permanent differences	70	129	157
Federal and state research credits—current year	(765)	(979)	(722)
Accrual (release) of uncertain tax positions	(21)	43	101
Expiration of statutes for uncertain tax positions	(953)	0	0
Impact of change in federal and state tax rate on revaluing deferred tax assets	110	(4)	56
Change in valuation allowance	2,331	4,687	756
Acquisition related adjustments	(485)	0	0
Other	(201)	(102)	(77)
Benefit for income taxes	$ (3,909)	$ (2,391)	$ (3,218)	$ (3,909)