|**Asset Retirement Obligations (Tables)**                            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                  
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|**Asset Retirement Obligation Disclosure [Abstract]**                |                                                                                                                                                                                                                                                                                                                                                                  
|Schedule of Change in Asset Retirement Obligation                    | Changes in AROs were as follows:Year Ended December 31,(millions)20172016Asset Retirement Obligations, Beginning Balance$935$989Liabilities Incurred9421Liabilities Settled(82)(120)Revision of Estimate(65)(3)Reclassification to Liabilities Associated with Assets Held for Sale(54)—Accretion Expense4748Asset Retirement Obligations, Ending Balance$875$935
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|                                                                     | Year Ended December 31,                                                                                                                                                                                                                                                                                                                                          
|(millions)                                                           | 2017                                                                                                                                                                                                                                                                                                                                                             
|Asset Retirement Obligations, Beginning Balance                      | $                                                                                                                                                                                                                                                                                                                                                                
|Liabilities Incurred                                                 | 94                                                                                                                                                                                                                                                                                                                                                               
|Liabilities Settled                                                  | (82                                                                                                                                                                                                                                                                                                                                                              
|Revision of Estimate                                                 | (65                                                                                                                                                                                                                                                                                                                                                              
|Reclassification to Liabilities Associated with Assets Held for Sale | (54                                                                                                                                                                                                                                                                                                                                                              
|Accretion Expense                                                    | 47                                                                                                                                                                                                                                                                                                                                                               
|Asset Retirement Obligations, Ending Balance                         | $                                                                                                                                                                                                                                                                                                                                                                

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duration X- DefinitionTabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation.
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