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Shareholders' Equity - Restricted Stock Awards (Details) - Restricted Stock - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Unvested Restricted Stock Outstanding        
Balance at the beginning of the period (in shares) 1.2 1.4 1.5  
Granted (in shares) 0.0 0.1 0.2  
Vested (in shares) (0.2) (0.2) (0.2)  
Forfeited (in shares) (0.7) (0.1) (0.1)  
Balance at the end of the period (in shares) 0.3 1.2 1.4 1.5
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]        
Balance at the beginning of the period (in dollars per share) $ 43.10 $ 42.69 $ 41.18  
Granted (in dollars per share) 0 42.92 49.95  
Vested (in dollars per share) 43.69 40.40 39.63  
Forfeited (in dollars per share) 41.50 43.30 41.84  
Balance at the end of the period (in dollars per share) $ 45.80 $ 43.10 $ 42.69 $ 41.18
Aggregate Intrinsic Value $ 9.7 $ 55.7 $ 54.9 $ 70.2
Weighted Average Remaining Vesting Term 27 days 24 days 1 year 4 months 24 days 2 years 1 month 6 days
Aggregate value of awards vested $ 7.8 $ 9.6 $ 9.9