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Stock-Based Compensation Plans (Summary Of Restricted Stock Activity) (Details) - Restricted Stock [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding, Nonvested as of December 31 137,503    
Vested (14,633)    
Forfeited (152)    
Outstanding, Nonvested as of December 31 122,718 137,503  
Weighted Average Grant-Date Fair Value Per Share, Non-vested as of December 31 $ 125.04    
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Granted   $ 441.54 $ 317.87
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Vested 182.61    
Weighted Average Grant-Date Fair Value Per Share, Nonvested, Forfeited 362.80    
Weighted Average Grant-Date Fair Value Per Share, Non-vested as of December 31 $ 117.88 $ 125.04