|**Asset Retirement Obligations (Tables)**             | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                           
|----------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Nine Months Ended September 30,(millions)20182017Asset Retirement Obligations, Beginning Balance$875$935Liabilities Incurred1683Liabilities Settled(309)(53)Revisions of Estimates67(56)Accretion Expense(1)2535Asset Retirement Obligations, Ending Balance$674$944(1)Accretion expense is included in depreciation, depletion and amortization (DD&A)expense in the consolidated statements ofoperations.
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|                                                      | Nine Months Ended September 30,                                                                                                                                                                                                                                                                                                                                                                                                          
|(millions)                                            | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Liabilities Incurred                                  | 16                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Liabilities Settled                                   | (309                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Revisions of Estimates                                | 67                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Accretion Expense(1)                                  | 25                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                                                                        

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