Stock Options Activity (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Shares
Beginning balance (in shares)	3,223,742
Granted (in shares)	647,767
Exercised (in shares)	(1,085,455)
Forfeited/Expired (in shares)	(73,136)
Ending balance (in shares)	2,712,918	3,223,742
Vested and expected to vest (in shares)	2,119,596
Exercisable (in shares)	1,237,438
Weighted Average Price
Beginning Balance, Weighted Average Price (in dollars per share)	$ 33.16
Granted, Weighted Average Price (in dollars per share)	$ 50.58
Exercised, Weighted Average Price (in dollars per share)	$ 28.14
Forfeited/Expired, Weighted Average Price (in dollars per share)	$ 42.56
Ending Balance, Weighted Average Price (in dollars per share)	$ 39.07	$ 33.16
Vested and expected to vest , Weighted Average Price (in dollars per share)	$ 38.97
Exercisable at March 31, 2013, Weighted Average Price (in dollars per share)	$ 32.11
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term, Ending	6 years 11 months 19 days	6 years 4 months 13 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 9 months 18 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 1 month 24 days
Aggregate Intrinsic Value
Stock options outstanding, Beginning aggregate intrinsic value	$ 43,106,153
Stock options outstanding, Ending aggregate intrinsic value	71,000,856	43,106,153
Stock options, vested and expected to vest aggregate intrinsic value	55,701,676
Stock options, exercisable aggregate intrinsic value	$ 41,013,330