|**Asset Retirement Obligations (Tables)**                            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|**Asset Retirement Obligation Disclosure [Abstract]**                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Schedule of Change in Asset Retirement Obligation                    | ARO consists 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:Year Ended December 31,(millions)20182017Asset Retirement Obligations, Beginning Balance$875$935Liabilities Incurred2594Liabilities Settled(345)(82)Revisions of Estimates293(65)Reclassification to Liabilities Associated with Assets Held for Sale(1)(54)Accretion Expense3347Asset Retirement Obligations, Ending Balance$880$875
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|                                                                     | Year Ended December 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(millions)                                                           | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Asset Retirement Obligations, Beginning Balance                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Liabilities Incurred                                                 | 25                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Liabilities Settled                                                  | (345                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Revisions of Estimates                                               | 293                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Reclassification to Liabilities Associated with Assets Held for Sale | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Accretion Expense                                                    | 33                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Asset Retirement Obligations, Ending Balance                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

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