Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of Shares
Beginning Balance	6,505,664	6,747,303	7,147,303
Granted	427,800	1,099,300	2,649,900
Exercised	(3,249,160)	(613,138)	(570,168)
Cancelled	(202,835)	(727,801)	(2,479,732)
Ending Balance	3,481,469	6,505,664	6,747,303
Vested and unvested expected to vest at end of period	3,443,414
Weighted Average Exercise Price
Beginning Balance	$ 4.07	$ 3.81	$ 4.11
Granted	10.47	4.95	3.28
Exercised	3.95	2.26	1.68
Cancelled	4.36	4.54	4.58
Ending Balance	$ 4.96	$ 4.07	$ 3.81
Options exercisable, at end of period	2,120,834	3,781,162
Options available for future grant	2,768,230
Vested and unvested expected to vest at end of period	$ 4.24
Options exercisable, at end of period	$ 4.24	$ 4.10
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 11 months 23 days
Vested and unvested expected to vest at end of period	5 years 10 months 2 days
Options exercisable, at end of period	5 years 10 months 2 days	5 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 12,601
Vested and unvested expected to vest at end of period	12,463
Options exercisable, at end of period	$ 8,622	$ 4,130