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Savings Plans, Pension Plans and Other Postretirement Employee Benefits (Weighted Average Assumptions Used to Determine the Net Periodic Cost (Benefit) for Non-Deltic and Deltic Plans) (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plans [Member] | Non-Deltic Plans [Member]      
Discount rate 3.40% 4.40% 3.85%
Expected return on plan assets 5.75% 6.25% 6.25%
Rate of salaried compensation increase 3.00% 3.00% 3.00%
Pension Plans [Member] | Deltic Plans [Member]      
Discount rate 3.40% 4.40% 4.30%
Expected return on plan assets 5.75% 6.25% 6.25%
Rate of salaried compensation increase 4.00% 4.00% 4.00%
OPEB [Member] | Non-Deltic Plans [Member]      
Discount rate 3.40% 4.40% 3.65%
OPEB [Member] | Deltic Plans [Member]      
Discount rate 3.40% 4.40% 4.30%