Stock Options Activity (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Shares
Outstanding at January 1, 2013	3,223,742
Granted	619,385
Exercised	(813,702)
Forfeited/Expired	(62,421)
Outstanding at June 30, 2013	2,967,004	3,223,742
Vested and expected to vest at June 30, 2013	2,323,450
Exercisable at June 30, 2013	1,501,547
Weighted Average Price
Stock Options Outstanding, Weighted Average Price, Beginning	$ 33.16
Granted, Weighted Average Price	$ 50.27
Exercised, Weighted Average Price	$ 28.60
Forfeited/Expired, Weighted Average Price	$ 42.10
Stock Options Outstanding, Weighted Average Price, Ending	$ 37.79	$ 33.16
Vested and expected to vest at June 30, 2013, Weighted Average Price	$ 37.33
Exercisable at June 30, 2013, Weighted Average Price	$ 31.12
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term, Ending	6 years 11 months 12 days	6 years 4 months 13 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest at June 30, 2013	6 years 8 months 16 days
Weighted-Average Remaining Contractual Term, Exercisable at June 30, 2013	5 years 2 months 27 days
Aggregate Intrinsic Value
Stock options outstanding, Beginning aggregate intrinsic value	$ 43,106,153
Stock options outstanding, Ending aggregate intrinsic value	47,524,458	43,106,153
Stock options, vested and expected to vest aggregate intrinsic value	38,284,375
Stock options, exercisable aggregate intrinsic value	$ 34,065,352