Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Beginning Balance	5,277,085	3,852,135	3,465,335
Granted	2,880,691	1,744,950	688,800
Exercised	(581,105)	(104,167)	(180,000)
Cancelled	(1,733,792)	(215,833)	(122,000)
Ending Balance	5,842,879	5,277,085	3,852,135
Options exercisable, at end of period	2,765,357	3,099,935
Options available for future grant	2,503,508
Weighted Average Exercise Price
Beginning Balance	$ 4.24	$ 5.12	$ 5.07
Granted	3.40	2.35	5.27
Exercised	3.30	2.30	3.67
Cancelled	4.21	5.72	6.64
Ending Balance	3.21	4.24	$ 5.12
Options exercisable, at end of period	$ 4.39	$ 5.15
Weighted Average Contractual Term (Years)
Outstanding, at end of period	8 years 25 days
Options exercisable, at end of period	6 years 8 months 12 days	4 years 11 months 4 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 7,482
Options exercisable, at end of period	$ 3,603	$ 88