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Employee Stock and Incentive Plans - Non-Qualified Stock Options (Details) - Non-Qualified Stock Options [Member] - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2008
Number of Options [Roll Forward]        
Outstanding as of January 1 39,654 66,279 108,299  
Granted [1] 0 0 0  
Exercised (35,297) (24,456) (42,020)  
Forfeited 0 (2,169) 0  
Outstanding as of December 31 4,357 39,654 66,279  
Exercisable as of December 31 4,357 39,654 66,279  
Weighted-Average Exercise Price [Abstract]        
Outstanding as of January 1 $ 44.39 $ 44.39 $ 44.10  
Granted [1] 0.00 0.00 0.00  
Exercised 44.89 44.52 43.65  
Forfeited 0.00 42.93 0.00  
Outstanding as of December 31 40.29 44.39 44.39  
Exercisable as of December 31 $ 40.29 $ 44.39 $ 44.39  
Weighted-Average Grant Date Intrinsic Value       $ 6.18
Cash Received from Non-Qualified Stock Options Exercised $ 1.6      
Total Intrinsic Value of Options Exercised $ 0.5 $ 0.2 $ 0.4  
[1] Stock options have not been granted since 2008. The weighted-average grant-date intrinsic value of options granted in 2008 was $6.18.