Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013
Number of Shares
Outstanding, Beginning Balance	2,968,582
Granted	18,359
Exercised	(169,605)
Expired	(429)
Outstanding, Ending Balance	2,816,907
Number of Shares Exercisable, Ending Balance	1,683,640
Weighted-Average Exercise Price
Beginning Balance	$ 27.15
Granted	$ 47.63
Exercised	$ 22.78
Expired	$ 28.94
Ending Balance	$ 27.55
Weighted Average Exercise Price Exercisable, Ending Balance	$ 24.63
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	5 years 2 months 23 days
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	4 years 5 months 16 days
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of year	$ 60,039,934
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year	$ 40,792,908