|**Asset Retirement Obligations**                                     | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
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|**Asset Retirement Obligation Disclosure [Abstract]**                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Asset Retirement Obligations                                         | Note 8. Asset Retirement ObligationsAsset retirement obligations (ARO) consist primarily of estimated costs of dismantlement, removal, site reclamation and similar activities associated with our oil and gas properties. Changes in ARO are as follows:Three Months Ended March 31,(millions)20182017Asset Retirement Obligations, Beginning Balance$875$935Liabilities Incurred21Liabilities Settled(20)(9)Revisions of Estimates(11)(7)Reclassification to Liabilities Associated with Assets Held for Sale(227)—Accretion Expense(1)912Asset Retirement Obligations, Ending Balance$628$932(1)Accretion expense is included in depreciation, depletion and amortization (DD&A)expense in the consolidated statements ofoperations.For theThree Months Ended March 31, 2018In 2018, we transferred$227 millionof ARO liabilities related to Gulf of Mexico properties to liabilities associated with assets held for sale. Liabilities settled include$20 millionrelated to abandonment of US onshore properties, primarily in the DJ Basin. Revisions of estimates relate to decreases in cost and timing estimates of$11 millionassociated with the North Sea abandonment project.For theThree Months Ended March 31, 2017Liabilities incurred were due to new wells and facilities placed into service for US onshore. Liabilities settled primarily related to US onshore property abandonments. Revisions of estimates related to changes in Gulf of Mexico cost estimates.
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|                                                                     | Three Months Ended March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(millions)                                                           | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Asset Retirement Obligations, Beginning Balance                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Liabilities Incurred                                                 | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Liabilities Settled                                                  | (20                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Revisions of Estimates                                               | (11                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Reclassification to Liabilities Associated with Assets Held for Sale | (227                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Accretion Expense(1)                                                 | 9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Asset Retirement Obligations, Ending Balance                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|(1)                                                                  | Accretion expense is included in depreciation, depletion and amortization (DD&A)expense in the consolidated statements ofoperations.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

X- ReferencesNo definition available.
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duration X- DefinitionThe entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671
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