Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income taxes computed at statutory rates	$ (7,369)	$ (6,081)	$ (5,159)
Increases (decreases) resulting from:
State income taxes, net of federal tax benefit	(1,219)	(1,298)	(700)
Impact of federal graduated rates		(4)
Federal and state research and experimentation credits	(1,112)	(917)	(563)
Change in valuation allowance	9,468	8,132	6,916
Other	242	234	158
Total tax provision (benefit)	$ 10	$ 66	$ 652
Income taxes computed at statutory rates, Percentage	34.00%	34.00%	34.00%
Increases (decreases) resulting from:
State income taxes, net of federal tax benefit, Percentage	6.00%	7.00%	5.00%
Impact of federal graduated rates, Percentage	0.00%	0.00%
Federal and state research and experimentation credits, Percentage	5.00%	5.00%	4.00%
Change in valuation allowance, Percentage	(44.00%)	(45.00%)	(46.00%)
Other, Percentage	(1.00%)	(1.00%)	(1.00%)
Total, Percentage	0.00%	0.00%	(4.00%)