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Asset Retirement Obligations and Accrued Environmental Costs (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Site Contingency [Line Items]    
Accrued environmental costs $ 496 $ 485
Maximum period over which accrued environmental costs are expected to be paid, years 30 years  
Domestic refineries and underground storage tanks    
Site Contingency [Line Items]    
Accrued environmental costs $ 268 270
Nonoperator sites    
Site Contingency [Line Items]    
Accrued environmental costs 178 168
Other sites    
Site Contingency [Line Items]    
Accrued environmental costs 50 $ 47
Acquired through Business Combination    
Site Contingency [Line Items]    
Accrued environmental costs 248  
Expected future undiscounted payments related to the portion of the accrued environmental costs that have been discounted    
Expected future undiscounted payments, due in 2017 25  
Expected future undiscounted payments, due in 2018 26  
Expected future undiscounted payments, due in 2019 22  
Expected future undiscounted payments, due in 2020 16  
Expected future undiscounted payments, due in 2021 15  
Expected future undiscounted payments, due for all future years after 2021 $ 212  
Weighted Average | Acquired through Business Combination    
Site Contingency [Line Items]    
Accrued environmental costs, discount rate 5.00%