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Benefit Plans, Assumptions (Details) - SERP [Member]
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Assumptions Used to Determine Net Pension Costs [Abstract]      
Discount rate 3.29% 4.36% 3.68%
Rate of compensation increase [1]
Assumptions Used to Determine Benefit Obligations [Abstract]      
Discount rate 2.41% 3.29%  
Rate of compensation increase [1]  
[1] n/a