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Stock Award Plans, Employee Plan RSU Activity (Details) - RSUs [Member] - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Unvested RSU Shares [Roll Forward]      
Vested (in shares)   0 0
Employee Stock Award Plan [Member]      
Unvested RSU Shares [Roll Forward]      
Nonvested balance beginning of period (in shares) 141,055    
Granted (in shares) 146,650   0
Vested (in shares) (25,484)    
Forfeited (in shares) (7,054)    
Nonvested balance end of period (in shares) 255,167 141,055  
Weighted Average Grant Date Fair Value Per Share [Abstract]      
Nonvested balance beginning of period (in dollars per share) $ 75.59    
Granted (in dollars per share) 74.46 $ 75.59  
Vested (in dollars per share) 75.60    
Forfeited (in dollars per share) 74.93    
Nonvested balance end of period (in dollars per share) $ 74.96 $ 75.59