Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Beginning Balance	5,842,879	5,277,085	3,852,135
Granted	2,222,368	2,880,691	1,744,950
Exercised	(338,333)	(581,105)	(104,167)
Cancelled	(894,528)	(1,733,792)	(215,833)
Ending Balance	6,832,386	5,842,879	5,277,085
Options exercisable, at end of period	3,596,315	2,765,357
Options available for future grant	5,714,648
Weighted Average Exercise Price
Beginning Balance	$ 3.21	$ 4.24	$ 5.12
Granted	3.39	3.40	2.35
Exercised	2.01	3.30	2.35
Cancelled	4.22	4.21	5.72
Ending Balance	3.81	3.21	$ 4.24
Options exercisable, at end of period	$ 4.17	$ 4.39
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 11 months 8 days
Options exercisable, at end of period	6 years 10 months 24 days	6 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 812
Options exercisable, at end of period	$ 598	$ 3,603