Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes computed at statutory rates	$ 6,818	$ 8,834
(Increases) decreases resulting from:
State income taxes, net of federal tax benefit	2,054	1,576
Federal and state research and experimentation credits	1,064	1,081
Change in valuation allowance	(10,024)	(1,216)
Impact of expired tax positions	1	(7)
Impact of recent U.S. tax reform	0	(10,529)
Other	48	467
Total tax benefit (provision)	$ (39)	$ 206
Income taxes computed at statutory rates, Percentage	(21.00%)	(34.00%)
(Increases) decreases resulting from:
State income taxes, net of federal tax benefit, Percentage	(6.00%)	(6.00%)
Federal and state research and experimentation credits, Percentage	(4.00%)	(4.00%)
Change in valuation allowance, Percentage	31.00%	5.00%
Impact of expired tax positions, Percentage	(0.00%)	(0.00%)
Impact of recent U.S. tax reform, Percentage	(0.00%)	40.00%
Other, Percentage	(0.00%)	(2.00%)
Total, Percentage	(0.00%)	(1.00%)

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