Transaction under Stock Option Plan (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Aug. 13, 2007
Number of Shares
Beginning Balance	5,138,486	4,566,935	3,534,686
Granted	2,309,650	1,894,300	1,293,000
Exercised	(8,300)	(479,666)	(196,167)
Cancelled	(292,533)	(843,083)	(64,584)
Ending Balance	7,147,303	5,138,486	4,566,935
Vested and unvested expected to vest at end of period	7,096,125
Options exercisable, at end of period	3,683,786	2,911,186		25,111
Options available for future grant	1,706,020
Weighted Average Exercise Price
Beginning Balance	$ 4.08	$ 2.82	$ 2.82
Granted	$ 4.36	$ 5.65	$ 4.55
Exercised	$ 3.61	$ 2.04	$ 1.19
Cancelled	$ 5.70	$ 5.01	$ 4.36
Ending Balance	$ 4.11	$ 4.08	$ 2.82
Vested and unvested expected to vest at end of period	$ 3.56
Options exercisable, at end of period	$ 3.56	$ 3.21
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 3 months 4 days
Vested and unvested expected to vest at end of period	5 years 3 months 11 days
Options exercisable, at end of period	5 years 3 months 11 days	5 years 6 months 7 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 3,973
Vested and unvested expected to vest at end of period	3,944
Options exercisable, at end of period	$ 3,972	$ 4,232