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Employee Benefit Plans - Weighted average assumptions used to determine projected benefit obligation and net benefit cost (Details) - Retirement Income Plan
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Projected Benefit Obligation:      
Discount rate 2.50% 3.60% 4.65%
Rate of compensation increase 0.00% 0.00% 0.00%
Net Benefit Cost:      
Discount rate 3.60% 4.65% 4.00%
Expected return on plan assets 4.00% 7.00% 7.00%
Rate of compensation increase 0.00% 0.00% 0.00%