Asset Retirement Obligations and Accrued Environmental Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Site Contingency [Line Items]
Decrease in total accrued environmental	$ 6
Accrued environmental costs	441	$ 447
Domestic Refineries and Underground Sites
Site Contingency [Line Items]
Accrued environmental costs	240
Nonoperator sites
Site Contingency [Line Items]
Accrued environmental costs	147
Other sites
Site Contingency [Line Items]
Accrued environmental costs	54
Acquired through Business Combination
Site Contingency [Line Items]
Accrued environmental costs	246
Expected future undiscounted payments related to the portion of the accrued environmental costs that have been discounted
Expected future undiscounted payments, due in 2020	26
Expected future undiscounted payments, due in 2021	24
Expected future undiscounted payments, due in 2022	23
Expected future undiscounted payments, due in 2023	19
Expected future undiscounted payments, due in 2024	16
Expected future undiscounted payments, due for all future years after 2024	$ 206
Weighted Average | Acquired through Business Combination
Site Contingency [Line Items]
Accrued environmental costs, discount rate, percent	5.00%