Stock Option Activity under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Number of Shares
Beginning Balance	3,852,135
Granted	7,500
Exercised	0
Cancelled	(36,667)
Ending Balance	3,822,968	3,852,135
Options exercisable, at end of period	2,951,502	2,925,502
Options available for future grant	403,962
Weighted Average Exercise Price
Beginning Balance	$ 5.12
Granted	4.27
Exercised	0
Cancelled	5.93
Ending Balance	5.08	$ 5.12
Options exercisable, at end of period	$ 4.83	$ 5.12
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 2 months 12 days
Options exercisable, at end of period	5 years 4 months 9 days	5 years 6 months 29 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 1,020
Options exercisable, at end of period	$ 1,019	$ 1,152