Stock Option Activity Under Stock Option Plan (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Number of Shares
Beginning Balance	6,747,303	7,147,303	5,138,486
Granted	1,099,300	2,649,900	2,309,650
Exercised	(613,138)	(570,168)	(8,300)
Cancelled	(727,801)	(2,479,732)	(292,533)
Ending Balance	6,505,664	6,747,303	7,147,303
Vested and unvested expected to vest at end of period	6,485,430
Weighted Average Exercise Price
Beginning Balance	$ 3.81	$ 4.11	$ 4.08
Granted	$ 4.95	$ 3.28	$ 4.36
Exercised	$ 2.26	$ 1.68	$ 3.61
Cancelled	$ 4.54	$ 4.58	$ 5.70
Ending Balance	$ 4.07	$ 3.81	$ 4.11
Vested and unvested expected to vest at end of period	$ 4.10
Options exercisable, at end of period	$ 4.10	$ 4.01
Options exercisable, at end of period	3,781,162	3,471,935
Options available for future grant	4,585,769
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 2 months 9 days
Vested and unvested expected to vest at end of period	5 years 9 months 18 days
Options exercisable, at end of period	5 years 9 months 18 days	5 years 11 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 9,084
Vested and unvested expected to vest at end of period	9,056
Options exercisable, at end of period	$ 4,130	$ 2,654