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Stock-Based Compensation Plans Restricted Stock Awards (Details) - Restricted Stock Awards [Member] - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Shares [Abstract]      
Non-vested at beginning of year 266,370    
Granted 106,799    
Vested (67,806)    
Forfeited (16,347)    
Non-vested at end of year 289,016 266,370  
Weighted Average Grant Date Fair Value [Abstract]      
Non-vested at beginning of year $ 64.76    
Granted 73.07 $ 57.11 $ 79.30
Vested 78.33    
Forfeited 63.07    
Non-vested at end of year 64.74 64.76  
Weighted average grant date fair value granted during year $ 73.07 $ 57.11 $ 79.30
Total fair value of restricted stock awards that vested during year $ 5.3 $ 6.5 $ 5.2