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Stock Award Plans, Employee Plan Restricted Stock Award Activity (Details) - Employee Stock Award Plan [Member] - Restricted Stock [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Unvested RSU Shares [Roll Forward]      
Nonvested balance beginning of period (in shares) 134,669    
Vested (in shares) (48,450)    
Forfeited (in shares) (2,317)    
Nonvested balance end of period (in shares) 83,902 134,669  
Weighted Average Grant Date Fair Value Per Share [Abstract]      
Nonvested balance beginning of period (in dollars per share) $ 63.28    
Vested (in dollars per share) 63.24    
Forfeited (in dollars per share) 62.46    
Nonvested balance end of period (in dollars per share) $ 63.33 $ 63.28  
Granted (in shares) 0 0 0