Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Number of Shares
Beginning Balance	5,277,085
Granted	1,496,333	7,500
Cancelled	(1,357,333)
Ending Balance	5,416,085		5,277,085
Options exercisable, at end of period	2,213,008		3,099,935
Options available for future grant	3,996,638
Weighted Average Exercise Price
Beginning Balance	$ 4.24
Granted	2.24
Cancelled	4.33
Ending Balance	3.66		$ 4.24
Options exercisable, at end of period	$ 5.14		$ 5.15
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 9 months 25 days
Options exercisable, at end of period	5 years 3 months 29 days		4 years 11 months 4 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 5,494
Options exercisable, at end of period	$ 1,025		$ 88