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Stock-based Compensation (Schedule of Restricted Stock And Restricted Stock Unit Activity) (Details) - Restricted Stock And Restricted Stock Units [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of Shares      
Unvested stock awards at January 1, 2020 21,111    
Granted in 2020 6,779    
Vested in 2020 4,708    
Forfeited in 2020 2,268    
Unvested stock awards at December 31, 2020 20,914 21,111  
Weighted Average Grant-Date Fair Value      
Unvested stock awards at January 1, 2020 $ 427.14    
Granted in 2020 414.33 $ 440.42 $ 415.98
Vested in 2020 428.54    
Forfeited in 2020 428.07    
Unvested stock awards at December 31, 2020 $ 422.57 $ 427.14