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Benefit Plans - Key Assumptions for Projected Benefit Obligation and Net Periodic Benefit Cost (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Wyoming Refining plan      
Projected benefit obligation:      
Discount rate 2.65% 3.30% 4.20%
Rate of compensation increase 3.00% 3.00% 3.00%
Net periodic benefit costs:      
Discount rate 3.30% 4.20% 3.65%
Expected long-term rate of return 6.25% 6.50% 6.50%
Rate of compensation increase 3.00% 3.00% 3.00%
U.S. Oil plan      
Projected benefit obligation:      
Discount rate 2.35% 3.10% 0.00%
Rate of compensation increase 3.00% 3.00% 0.00%
Net periodic benefit costs:      
Discount rate 3.10% 4.10% 0.00%
Expected long-term rate of return 6.00% 6.00% 0.00%
Rate of compensation increase 3.00% 3.00% 0.00%