Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Number of Shares
Beginning Balance			5,842,879
Granted	153,000	199,422	1,049,000	1,695,755
Exercised			(8,166)
Cancelled			(322,200)
Ending Balance	6,561,513		6,561,513		5,842,879
Options exercisable, at end of period	3,453,006		3,453,006		2,765,357
Options available for future grant	6,032,404		6,032,404
Weighted Average Exercise Price
Beginning Balance			$ 3.21
Granted			4.01
Exercised			1.87
Cancelled			3.49
Ending Balance	$ 3.96		3.96		$ 3.21
Options exercisable, at end of period	$ 3.83		$ 3.83		$ 4.39
Weighted Average Contractual Term (Years)
Outstanding, at end of period			7 years 10 months 24 days
Options exercisable, at end of period			6 years 8 months 4 days		6 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 2,900		$ 2,900
Options exercisable, at end of period	$ 1,700		$ 1,700		$ 3,603