Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012
Number of Shares
Outstanding, Beginning Balance	3,490,056
Granted	108,350
Exercised	(777,018)
Forfeited	(94,230)
Expired	(4,000)
Outstanding, Ending Balance	2,723,158
Number of Shares Exercisable, Ending Balance	1,694,050
Weighted-Average Exercise Price
Beginning Balance	$ 23.28
Granted	$ 41.28
Exercised	$ 19.38
Forfeited	$ 21.60
Expired	$ 10.19
Ending Balance	$ 25.18
Weighted Average Exercise Price Exercisable, Ending Balance	$ 24.37
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	5 years 7 days
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	4 years 4 months 10 days
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of year	$ 46,605,039
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year	$ 30,310,213