|**Asset Retirement Obligations (Tables)**             | 6 Months Ended                                                                                                                                                                                                                                                                                                        
|----------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                       
|Changes in Asset Retirement Obligations               | Changes in asset retirement obligations were as follows:Six  Months Ended June 30,20122011(millions)Asset Retirement Obligations, Beginning Balance$377$253Liabilities Incurred231Liabilities Settled(2)(12)Revision of Estimate206Accretion Expense1410Other(89)—Asset Retirement Obligations, Ending Balance$343$258
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|                                                      |                                                                                                                                                                                                                                                                                                                       
|                                                      | Six  Months Ended June 30,                                                                                                                                                                                                                                                                                            
|                                                      | 2012                                                                                                                                                                                                                                                                                                                  
|(millions)                                            |                                                                                                                                                                                                                                                                                                                       
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                     
|Liabilities Incurred                                  | 23                                                                                                                                                                                                                                                                                                                    
|Liabilities Settled                                   | (2                                                                                                                                                                                                                                                                                                                    
|Revision of Estimate                                  | 20                                                                                                                                                                                                                                                                                                                    
|Accretion Expense                                     | 14                                                                                                                                                                                                                                                                                                                    
|Other                                                 | (89                                                                                                                                                                                                                                                                                                                   
|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.+ 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+ Details**Name:**
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