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Stock-Based Compensation Plans Restricted Stock Units (Details) - Restricted stock units (RSUs) [Member] - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units [Abstract]      
Non-vested at beginning of year 62,701    
Granted 28,668    
Vested (26,170)    
Forfeited (10,361)    
Non-vested at end of year 54,838 62,701  
Weighted Average Grant Date Fair Value [Abstract]      
Non-vested at beginning of year $ 65.35    
Granted 73.18 $ 57.11 $ 70.64
Vested 70.03    
Forfeited 67.95    
Non-vested at end of year 66.72 65.35  
Weighted average grant date fair value granted during year $ 73.18 $ 57.11 $ 70.64
Total fair value of restricted stock units that vested during year $ 1.8 $ 1.8 $ 4.2