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Income Tax - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Income Tax Examination [Line Items]        
Income tax expense $ 86.6 $ 77.5 $ 162.2 $ 156.5
Effective tax rate 32.50% 33.80% 32.30% 34.10%