Summary of Stock Options Outstanding by Exercise Price Range (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number of Options	9,521,970
Options Outstanding, Weighted-Average Exercise Price	$ 8.13
Options Outstanding, Weighted-Average Remaining Contractual Term	8 years
Options Vested and Exercisable, Number of Options	4,367,013
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 6.30
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	7 years 2 months 12 days
$0.001 to 1.01
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, lower limit	$ 0.001
Range of Exercise Prices, upper limit	$ 1.01
Options Outstanding, Number of Options	1,381,244
Options Outstanding, Weighted-Average Exercise Price	$ 0.58
Options Outstanding, Weighted-Average Remaining Contractual Term	4 years 8 months 12 days
Options Vested and Exercisable, Number of Options	1,272,281
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 0.54
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	4 years 7 months 6 days
$2.35
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices	$ 2.35
Options Outstanding, Number of Options	1,181,704
Options Outstanding, Weighted-Average Exercise Price	$ 2.35
Options Outstanding, Weighted-Average Remaining Contractual Term	7 years 6 months
Options Vested and Exercisable, Number of Options	753,440
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 2.35
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	7 years 6 months
$6.79 to 8.92
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, lower limit	$ 6.790
Range of Exercise Prices, upper limit	$ 8.92
Options Outstanding, Number of Options	2,703,723
Options Outstanding, Weighted-Average Exercise Price	$ 7.87
Options Outstanding, Weighted-Average Remaining Contractual Term	8 years 2 months 12 days
Options Vested and Exercisable, Number of Options	1,249,747
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 7.83
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	8 years 1 month 6 days
$9.37 to 11.59
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, lower limit	$ 9.370
Range of Exercise Prices, upper limit	$ 11.59
Options Outstanding, Number of Options	2,628,609
Options Outstanding, Weighted-Average Exercise Price	$ 10.89
Options Outstanding, Weighted-Average Remaining Contractual Term	9 years 1 month 6 days
Options Vested and Exercisable, Number of Options	551,766
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 11.10
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	9 years 1 month 6 days
$11.61 to 27.03
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, lower limit	$ 11.610
Range of Exercise Prices, upper limit	$ 27.03
Options Outstanding, Number of Options	1,626,690
Options Outstanding, Weighted-Average Exercise Price	$ 14.75
Options Outstanding, Weighted-Average Remaining Contractual Term	8 years 10 months 24 days
Options Vested and Exercisable, Number of Options	539,779
Options Vested and Exercisable, Weighted-Average Exercise Price	$ 16.93
Options Vested and Exercisable, Weighted-Average Remaining Contractual Term	8 years 7 months 6 days