Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013
Number of Shares
Outstanding, Beginning Balance	2,968,582
Granted	84,705
Exercised	(242,379)
Forfeited	(19,722)
Expired	(429)
Outstanding, Ending Balance	2,790,757
Number of Shares Exercisable, Ending Balance	1,670,866
Weighted-Average Exercise Price
Beginning Balance	$ 27.15
Granted	$ 43.93
Exercised	$ 22.68
Forfeited	$ 33.83
Expired	$ 28.94
Ending Balance	$ 28.00
Weighted Average Exercise Price Exercisable, Ending Balance	$ 25.26
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	5 years 1 month 10 days
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	4 years 5 months 1 day
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of year	$ 51,594,113
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year	$ 35,448,044