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Share-Based Compensation Plans - Non-Employee Directors' Share Based Awards (Narrative) (Details) - Non-employee Directors' Share-Based Awards - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares outstanding 222,251    
Shares granted 24,110 17,900 25,500
Weighted average fair value, granted (in dollars per share) $ 75.52 $ 89.78 $ 58.98