Benefit Plans - Asset Allocation (Details)	Dec. 31, 2019
U.S. Oil plan
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	100.00%
Actual plan asset allocations	100.00%
U.S. Oil plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	62.00%
U.S. Oil plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	38.00%
U.S. Oil plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	0.00%
Wyoming Refining plan
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	100.00%
Actual plan asset allocations	100.00%
Wyoming Refining plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	54.00%
Actual plan asset allocations	56.00%
Wyoming Refining plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	35.00%
Actual plan asset allocations	32.00%
Wyoming Refining plan | Real estate
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	11.00%
Actual plan asset allocations	12.00%
Minimum | U.S. Oil plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	40.00%
Minimum | U.S. Oil plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	20.00%
Minimum | U.S. Oil plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	0.00%
Maximum | U.S. Oil plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	80.00%
Maximum | U.S. Oil plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	60.00%
Maximum | U.S. Oil plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	30.00%