|**Asset Retirement Obligations (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                  
|----------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                 
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Three Months EndedMarch 31,20132012(millions)Asset Retirement Obligations, Beginning Balance$402$377Liabilities Incurred16Liabilities Settled(10)(2)Revision of Estimate(1)3Accretion Expense(1)77Asset Retirement Obligations, Ending Balance$399$391(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
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|                                                      |                                                                                                                                                                                                                                                                                                                                                                                 
|                                                      | Three Months EndedMarch 31,                                                                                                                                                                                                                                                                                                                                                     
|                                                      | 2013                                                                                                                                                                                                                                                                                                                                                                            
|(millions)                                            |                                                                                                                                                                                                                                                                                                                                                                                 
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                               
|Liabilities Incurred                                  | 1                                                                                                                                                                                                                                                                                                                                                                               
|Liabilities Settled                                   | (10                                                                                                                                                                                                                                                                                                                                                                             
|Revision of Estimate                                  | (1                                                                                                                                                                                                                                                                                                                                                                              
|Accretion Expense(1)                                  | 7                                                                                                                                                                                                                                                                                                                                                                               
|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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