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Share-Based Compensation - Summary of Recent Share-Based Compensation Expense (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Share-based compensation expense $ 29.3us-gaap_AllocatedShareBasedCompensationExpense $ 27.5us-gaap_AllocatedShareBasedCompensationExpense $ 25.4us-gaap_AllocatedShareBasedCompensationExpense
Tax benefit $ 11.3us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense $ 10.6us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense $ 9.6us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense