Stock Option Activity (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, Outstanding, December 31, 2011	5,475
Options, Granted	2,188
Options, Exercised	(907)
Options, Forfeited	(218)
Options, Outstanding, September 30, 2012	6,538	5,475
Options, Exercisable at December 31, 2011	2,233
Options, Vested and expected to vest at December 31, 2011	5,262
Options, Exercisable at September 30, 2012	2,339	2,233
Options, Vested and expected to vest at September 30, 2012	6,459	5,262
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted-Average Exercise Price, Outstanding, December 31, 2011	$ 6.04
Weighted-Average Exercise Price, Granted	$ 20.90
Weighted-Average Exercise Price, Exercised	$ 1.96
Weighted-Average Exercise Price, Forfeited	$ 13.93
Weighted-Average Exercise Price, Outstanding, September 30, 2012	$ 11.31	$ 6.04
Weighted-Average Exercise Price, Exercisable at December 31, 2011	$ 2.24
Weighted-Average Exercise Price, Vested and expected to vest at December 31, 2011	$ 5.83
Weighted-Average Exercise Price, Exercisable at September 31, 2012	$ 4.92	$ 2.24
Weighted-Average Exercise Price, Vested and expected to vest at September 30, 2012	$ 11.29	$ 5.83
Weighted-Average Remaining Contractual Term, Outstanding, December 31, 2011	8 years 4 months 24 days	8 years 3 months 18 days
Weighted-Average Remaining Contractual Term, Granted	9 years 7 months 6 days
Weighted-Average Remaining Contractual Term, Exercised	6 years
Weighted-Average Remaining Contractual Term, Outstanding, September 30, 2012	8 years 4 months 24 days	8 years 3 months 18 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2011	7 years 3 months 18 days	7 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest at December 31, 2011	8 years 4 months 24 days	8 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Exercisable at September 30, 2012	7 years 3 months 18 days	7 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest at September 30, 2012	8 years 4 months 24 days	8 years 2 months 12 days
Aggregate Intrinsic Value, Outstanding, December 31, 2011	$ 66,817
Aggregate Intrinsic Value, Outstanding, September 30, 2012	126,493	66,817
Aggregate Intrinsic Value, Exercisable at December 31, 2011	35,742
Aggregate Intrinsic Value, Vested and expected to vest at December 31, 2011	65,328
Aggregate Intrinsic Value, Exercisable at September 30, 2012	60,207	35,742
Aggregate Intrinsic Value, Vested and expected to vest at September 30, 2012	$ 125,080	$ 65,328