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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Unvested Restricted Stock Outstanding        
Balance at the beginning of the period (in shares) 1.5 1.4 1.1  
Granted (in shares) 0.2 0.3 0.3  
Vested (in shares) (0.2) (0.2)    
Forfeited (in shares) (0.1)      
Balance at the end of the period (in shares) 1.4 1.5 1.4 1.1
Balance at the beginning of the period (in dollars per share) $ 41.18 $ 40.07 $ 39.38  
Granted (in dollars per share) 49.95 44.18 41.83  
Vested (in dollars per share) 39.63 38.00 37.00  
Forfeited (in dollars per share) 41.84 39.58 38.89  
Balance at the end of the period (in dollars per share) $ 42.69 $ 41.18 $ 40.07 $ 39.38
Aggregate Intrinsic Value $ 54.9 $ 70.2 $ 59.3 $ 45.3
Weighted Average Remaining Vesting Term 1 year 4 months 24 days 2 years 1 month 6 days 2 years 8 months 12 days 3 years 4 months 24 days
Aggregate value of awards vested $ 9.9 $ 7.1 $ 1.7