Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income taxes computed at statutory rates	$ (8,834)	$ (7,369)	$ (6,081)
Increases (decreases) resulting from:
State income taxes, net of federal tax benefit	(1,576)	(1,219)	(1,298)
Impact of federal graduated rates	0	0	(4)
Federal and state research and experimentation credits	(1,081)	(1,112)	(917)
Change in valuation allowance	1,216	9,468	8,132
Impact of expired tax positions	7	(41)	0
Impact of recent U.S. tax reform	10,529	0	0
Other	(467)	283	234
Total tax provision (benefit)	$ (206)	$ 10	$ 66
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%	6.00%	7.00%
Impact of federal graduated rates, Percentage	0.00%	0.00%	0.00%
Federal and state research and experimentation credits, Percentage	4.00%	5.00%	5.00%
Change in valuation allowance, Percentage	(5.00%)	(44.00%)	(45.00%)
Impact of expired tax positions, Percentage	0.00%	0.00%	0.00%
Impact of recent U.S. tax reform, Percentage	(40.00%)	0.00%	0.00%
Other, Percentage	2.00%	(1.00%)	(1.00%)
Total, Percentage	1.00%	0.00%	0.00%

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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