Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Beginning Balance	3,852,135	3,465,335	3,481,468
Granted	1,744,950	688,800	362,800
Exercised	(104,167)	(180,000)	(234,833)
Cancelled	(215,833)	(122,000)	(144,100)
Ending Balance	5,277,085	3,852,135	3,465,335
Options exercisable, at end of period	3,099,935	2,925,502
Weighted Average Exercise Price
Beginning Balance	$ 5.12	$ 5.07	$ 4.96
Granted	2.35	5.27	6.40
Exercised	2.30	3.67	4.57
Cancelled	5.72	6.64	6.43
Ending Balance	4.24	5.12	$ 5.07
Options exercisable, at end of period	$ 5.15	$ 5.12
Options available for future grant	3,895,923
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 10 months 10 days
Options exercisable, at end of period	4 years 11 months 5 days	5 years 6 months 29 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 88
Options exercisable, at end of period	$ 88	$ 1,152