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Summary of Stock Option Activity (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Options      
Options, Outstanding, Beginning Balance 1,224,699 1,568,384 1,973,301
Options, Exercised (944,869) (343,685) (394,420)
Options, Forfeited (1,250)   (10,497)
Options, Outstanding and exercisable, Ending Balance 278,580 1,224,699 1,568,384
Weighted-Average Exercise Price      
Weighted-Average Exercise Price, Outstanding, Beginning Balance $ 21.64 $ 21.38 $ 20.92
Weighted-Average Exercise Price, Exercised $ 21.06 $ 20.34 $ 19.01
Weighted-Average Exercise Price, Forfeited $ 23.09   $ 24.62
Weighted-Average Exercise Price, Outstanding, Ending Balance   $ 21.64 $ 21.38
Weighted-Average Exercise Price, Outstanding and exercisable, Ending Balance $ 23.65    
Weighted-Average Remaining Contractual Term (years)      
Weighted-Average Remaining Contractual Term (years), Outstanding and exercisable, Ending Balance 1 year 3 months 18 days    
Aggregate Intrinsic Value      
Aggregate Intrinsic Value, Outstanding and exercisable, Ending Balance $ 4,130