Summary of Stock Option Activity under Plans and Related Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Number of Stock Options Outstanding
Beginning Balance	8,410,765
Granted	2,488,574
Exercised	(805,827)
Forfeited, cancelled or expired	(571,542)
Ending Balance	9,521,970
Options vested and exercisable at December 31, 2012	4,367,013
Options vested and expected to vest as of December 31, 2012	9,326,658
Weighted-Average Exercise Price
Beginning Balance	$ 6.76
Granted	$ 11.06
Exercised	$ 2.08
Forfeited, cancelled or expired	$ 9.17
Ending Balance	$ 8.13
Options vested and exercisable at December 31, 2012	$ 6.30
Options vested and expected to vest as of December 31, 2012	$ 7.76
Weighted-Average Remaining Contractual Term
Balance at December 31, 2012	8 years
Options vested and exercisable at December 31, 2012	7 years 2 months 12 days
Aggregate Intrinsic Value
Balance at December 31, 2012	$ 17,704	[1]
Options vested and exercisable at December 31, 2012	14,026	[1]
Options vested and expected to vest as of December 31, 2012	$ 19,887	[1]

[1] The aggregate intrinsic value represents the value by which the Company's closing stock price on the last trading day of the year ended December 31, 2012 exceeds the exercise price of the stock multiplied by the number of options outstanding or exercisable, excluding any that have a negative intrinsic value.