Stock Option Activity Under Stock Option Plan (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended
	Aug. 31, 2007	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Number of Shares
Beginning Balance		7,147,303	5,138,486	4,566,935
Granted		2,649,900	2,309,650	1,894,300
Exercised	(25,111)	(570,168)	(8,300)	(479,666)
Cancelled		(2,479,732)	(292,533)	(843,083)
Ending Balance		6,747,303	7,147,303	5,138,486
Vested and unvested expected to vest at end of period		6,711,969
Options exercisable, at end of period		3,471,935	3,683,786
Options available for future grant		416,964
Weighted Average Exercise Price
Beginning Balance		$ 4.11	$ 4.08	$ 2.82
Granted		$ 3.28	$ 4.36	$ 5.65
Exercised		$ 1.68	$ 3.61	$ 2.04
Cancelled		$ 4.58	$ 5.70	$ 5.01
Ending Balance		$ 3.81	$ 4.11	$ 4.08
Vested and unvested expected to vest at end of period		$ 4.01
Options exercisable, at end of period		$ 4.01	$ 3.56
Weighted Average Contractual Term (Years)
Outstanding, at end of period		7 years 2 months 1 day
Vested and unvested expected to vest at end of period		5 years 11 days
Options exercisable, at end of period		5 years 11 days	5 years 3 months 11 days
Aggregate Intrinsic Value
Outstanding, at end of period		$ 5,339
Vested and unvested expected to vest at end of period		5,311
Options exercisable, at end of period		$ 2,654	$ 3,972