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Stock-Based Compensation (Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation $ 20.6 $ 25.1 $ 24.0
Tax benefit related to stock-based compensation expense 7.8 9.6 9.2
Unrecognized compensation cost related to non-vested stock-based compensation arrangements $ 17.7    
Unrecognized compensation cost expected to be recognized over weighted average period (in years) 1 year 7 months 21 days    
Tax benefits related to stock options exercised and vesting of restricted stock $ 6.2 4.8 23.3
Restricted Stock Units (RSUs) [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation 14.2 2.3 2.7
Unrecognized compensation cost related to non-vested stock-based compensation arrangements 10.5    
Stock Options [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation 3.1 16.2 14.8
Unrecognized compensation cost related to non-vested stock-based compensation arrangements 3.0    
Restricted Stock [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Stock-based compensation 3.3 $ 6.6 $ 6.5
Unrecognized compensation cost related to non-vested stock-based compensation arrangements $ 4.2