Asset Retirement Obligations and Accrued Environmental Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Site Contingency [Line Items]
Decrease in total accrued environmental	$ 38
Accrued environmental costs	458	$ 496
Domestic Refineries and Underground Sites [Member]
Site Contingency [Line Items]
Total accrued environmental	222
Nonoperator sites
Site Contingency [Line Items]
Total accrued environmental	187
Other sites
Site Contingency [Line Items]
Total accrued environmental	49
Acquired through Business Combination
Site Contingency [Line Items]
Accrued environmental costs	268
Expected future undiscounted payments related to the portion of the accrued environmental costs that have been discounted
Expected future undiscounted payments, due in 2018	18
Expected future undiscounted payments, due in 2019	46
Expected future undiscounted payments, due in 2020	30
Expected future undiscounted payments, due in 2021	16
Expected future undiscounted payments, due in 2022	16
Expected future undiscounted payments, due for all future years after 2022	$ 216
Weighted Average | Acquired through Business Combination
Site Contingency [Line Items]
Accrued environmental costs, discount rate, percent	5.00%