Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Beginning Balance	3,465,335	3,481,468	6,505,663
Granted	688,800	362,800	427,800
Exercised	(180,000)	(234,833)	(3,249,160)
Cancelled	(122,000)	(144,100)	(202,835)
Ending Balance	3,852,135	3,465,335	3,481,468
Options exercisable, at end of period	2,925,502	2,671,835
Weighted Average Exercise Price
Beginning Balance	$ 5.07	$ 4.96	$ 4.07
Granted	5.27	6.40	10.47
Exercised	3.67	4.57	3.95
Cancelled	6.64	6.43	4.36
Ending Balance	5.12	5.07	$ 4.96
Options exercisable, at end of period	$ 5.12	$ 4.40
Options available for future grant	303,928
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 5 months 20 days
Options exercisable, at end of period	5 years 6 months 29 days	5 years 10 months 17 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 1,152
Options exercisable, at end of period	$ 1,152	$ 3,383