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Stock Plans (Summary of Stock Option Activity) (Details) - Employee Stock Option [Member] - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Outstanding January 1, 750 750 22,500  
Granted      
Exercised (750)   (750) (21,750)  
Forfeited      
Outstanding December 31,   0 750 22,500
Weighted-average grant date fair value, Outstanding January 1, $ 18.50 $ 18.50 $ 19.12  
Weighted-average grant date fair value - Granted      
Weighted-average grant date fair value - Exercised $ 18.50   18.50 19.14  
Weighted-average grant date fair value - Forfeited      
Weighted-average grant date fair value, Outstanding December 31,     $ 18.50 $ 19.12
Weighted-average remaining contractual term (years), Outstanding       2 months 12 days 1 year