Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Shares
Beginning Balance	3,481,468	6,505,663	6,747,302
Granted	362,800	427,800	1,099,300
Exercised	(234,833)	(3,249,160)	(613,138)
Cancelled	(144,100)	(202,835)	(727,801)
Ending Balance	3,465,335	3,481,468	6,505,663
Vested and unvested expected to vest at end of period	3,441,247
Weighted Average Exercise Price
Beginning Balance	$ 4.96	$ 4.07	$ 3.81
Granted	6.40	10.47	4.95
Exercised	4.57	3.95	2.26
Cancelled	6.43	4.36	4.54
Ending Balance	5.07	4.96	$ 4.07
Vested and unvested expected to vest at end of period	4.40
Options exercisable, at end of period	$ 4.40	$ 4.24
Options exercisable, at end of period	2,671,835	2,120,834
Options available for future grant	1,777,760
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 6 months 29 days
Vested and unvested expected to vest at end of period	5 years 10 months 17 days
Options exercisable, at end of period	5 years 10 months 17 days	5 years 10 months 2 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 3,407
Vested and unvested expected to vest at end of period	3,316
Options exercisable, at end of period	$ 3,383	$ 8,622