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Retirement Plans, Weighted-Average Assumption Used to Determine Benefit Obligation (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Pension Plan [Member]    
Weighted-average assumption used to determine benefit obligation at end of year [Abstract]    
Discount rate 2.80% [1] 3.50%
Health care cost trend rate [Abstract]    
Initial rate 0.00% 0.00%
Ultimate rate 0.00% 0.00%
Pension Plan [Member] | Higman [Member]    
Weighted-average assumption used to determine benefit obligation at end of year [Abstract]    
Discount rate [1] 2.90%  
SERP [Member]    
Weighted-average assumption used to determine benefit obligation at end of year [Abstract]    
Discount rate 2.80% 3.50%
Rate of compensation increase 0.00% 0.00%
Health care cost trend rate [Abstract]    
Initial rate 0.00% 0.00%
Ultimate rate 0.00% 0.00%
Other Postretirement Benefits [Member]    
Weighted-average assumption used to determine benefit obligation at end of year [Abstract]    
Discount rate 2.80% 3.50%
Rate of compensation increase 0.00% 0.00%
Health care cost trend rate [Abstract]    
Initial rate 6.50% 6.75%
Ultimate rate 5.00% 5.00%
Years to ultimate 2025 2025
[1] The 2020 discount rate was 2.8% for the Kirby pension plan and 2.9% for the Higman pension plan.