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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Number of Shares
Beginning Balance			5,277,085
Granted	199,422	198,000	1,695,755	205,500
Exercised			(332,416)
Cancelled			(1,562,833)
Ending Balance	5,077,591		5,077,591		5,277,085
Options exercisable, at end of period	2,378,158		2,378,158		3,099,935
Options available for future grant	4,069,982		4,069,982
Weighted Average Exercise Price
Beginning Balance			$ 4.24
Granted			2.54
Exercised			2.95
Cancelled			4.14
Ending Balance	$ 3.78		3.78		$ 4.24
Options exercisable, at end of period	$ 4.90		$ 4.90		$ 5.15
Weighted Average Contractual Term (Years)
Outstanding, at end of period			7 years 10 months 27 days
Options exercisable, at end of period			5 years 11 months 4 days		4 years 11 months 4 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 12,463		$ 12,463
Options exercisable, at end of period	$ 4,220		$ 4,220		$ 88