Stock Options Activity (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Shares
Outstanding at January 1, 2013	3,223,742
Granted	617,715
Exercised	(386,937)
Forfeited/Expired	(52,575)
Outstanding at March 31, 2013	3,401,945	3,223,742
Vested and expected to vest at March 31, 2013	2,987,418
Exercisable at March 31, 2013	1,913,459
Weighted Average Price
Stock Options Outstanding, Weighted Average Price, Beginning	$ 33.16
Granted, Weighted Average Price	$ 50.26
Exercised, Weighted Average Price	$ 27.87
Forfeited/Expired, Weighted Average Price	$ 42.01
Stock Options Outstanding, Weighted Average Price, Ending	$ 36.73	$ 33.16
Vested and expected to vest at March 31, 2013, Weighted Average Price	$ 36.33
Exercisable at March 31, 2013, Weighted Average Price	$ 30.63
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term, Ending	7 years	6 years 4 months 13 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest at March 31, 2013	6 years 10 months 6 days
Weighted-Average Remaining Contractual Term, Exercisable at March 31, 2013	5 years 5 months 9 days
Stock options outstanding, Beginning aggregate intrinsic value	$ 43,106,153
Stock options outstanding, Ending aggregate intrinsic value	56,781,822	43,106,153
Stock options, vested and expected to vest aggregate intrinsic value	51,055,880
Stock options, exercisable aggregate intrinsic value	$ 43,606,478