|**Asset Retirement Obligations (Tables)**             | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                             
|----------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                            
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Six Months EndedJune 30,(millions)20162015Asset Retirement Obligations, Beginning Balance$989$751Liabilities Incurred316Liabilities Settled(38)(15)Revision of Estimate479Accretion Expense(1)2521Asset Retirement Obligations, Ending Balance$983$852(1)Accretion expense is included in Depreciation, Depletion and Amortization (DD&A)expense in the consolidated statements ofoperations.
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|                                                      | Six Months EndedJune 30,                                                                                                                                                                                                                                                                                                                                                                                                   
|(millions)                                            | 2016                                                                                                                                                                                                                                                                                                                                                                                                                       
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                                                          
|Liabilities Incurred                                  | 3                                                                                                                                                                                                                                                                                                                                                                                                                          
|Liabilities Settled                                   | (38                                                                                                                                                                                                                                                                                                                                                                                                                        
|Revision of Estimate                                  | 4                                                                                                                                                                                                                                                                                                                                                                                                                          
|Accretion Expense(1)                                  | 25                                                                                                                                                                                                                                                                                                                                                                                                                         
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                                                          
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                            
|(1)                                                   | Accretion expense is included in Depreciation, Depletion and Amortization (DD&A)expense in the consolidated statements ofoperations.                                                                                                                                                                                                                                                                                       

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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.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859
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