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Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Number of Shares      
Outstanding, Beginning Balance 3,490,056 3,510,538 3,556,873
Granted 435,367 367,202 338,950
Exercised (847,301) (361,093) (235,986)
Forfeited (105,540) (26,591) (106,625)
Expired (4,000)   (42,674)
Outstanding, Ending Balance 2,968,582 3,490,056 3,510,538
Number of Shares Exercisable, Ending Balance 1,840,304    
Weighted-Average Exercise Price      
Beginning Balance $ 23.28 $ 21.55 $ 20.72
Granted $ 42.43 $ 28.95 $ 24.48
Exercised $ 19.74 $ 12.40 $ 13.34
Forfeited $ 22.19 $ 20.53 $ 18.11
Expired $ 10.19   $ 30.31
Ending Balance $ 27.15 $ 23.28 $ 21.55
Weighted Average Exercise Price Exercisable, Ending Balance $ 24.44    
Weighted-Average Remaining Contractual Term (Years)      
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of year 5 years 3 months 11 days    
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of year 4 years 5 months 27 days    
Aggregate Intrinsic Value of In-the-Money Options      
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of year $ 49,151,503    
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year $ 35,426,182