|**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 EndedSeptember 30,20132012(millions)Asset Retirement Obligations, Beginning Balance$402$377Liabilities Incurred424Liabilities Settled(15)(98)Revision of Estimate730Accretion Expense(1)2121Other(2)(34)Asset Retirement Obligations, Ending Balance$417$320(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
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|                                                      | Nine Months EndedSeptember 30,                                                                                                                                                                                                                                                                                                                                                                    
|                                                      | 2013                                                                                                                                                                                                                                                                                                                                                                                              
|(millions)                                            |                                                                                                                                                                                                                                                                                                                                                                                                   
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                                 
|Liabilities Incurred                                  | 4                                                                                                                                                                                                                                                                                                                                                                                                 
|Liabilities Settled                                   | (15                                                                                                                                                                                                                                                                                                                                                                                               
|Revision of Estimate                                  | 7                                                                                                                                                                                                                                                                                                                                                                                                 
|Accretion Expense(1)                                  | 21                                                                                                                                                                                                                                                                                                                                                                                                
|Other                                                 | (2                                                                                                                                                                                                                                                                                                                                                                                                
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                                 

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