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Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)
 9 Months Ended
Sep. 30, 2013
Number of Shares  
Outstanding, Beginning Balance 2,968,582
Granted 86,480
Exercised (410,263)
Forfeited (58,053)
Expired (429)
Outstanding, Ending Balance 2,586,317
Number of Shares Exercisable, Ending Balance 1,514,104
Weighted-Average Exercise Price  
Beginning Balance $ 27.15
Granted $ 44.08
Exercised $ 23.61
Forfeited $ 38.34
Expired $ 28.94
Ending Balance $ 28.03
Weighted Average Exercise Price Exercisable, Ending Balance $ 25.45
Weighted-Average Remaining Contractual Term (Years)  
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period 4 years 11 months 12 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 $ 67,331,959
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year $ 43,325,255