|**Asset Retirement Obligations (Tables)**             | 9 Months Ended                                                                                                                                                                                                                                                                                                             
|----------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                            
|Changes in Asset Retirement Obligations               | Changes in asset retirement obligations were as follows:Nine Months EndedSeptember 30,20122011(millions)Asset Retirement Obligations, Beginning Balance$377$253Liabilities Incurred242Liabilities Settled(98)(19)Revision of Estimate309Accretion Expense2115Other(34)—Asset Retirement Obligations, Ending Balance$320$260
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|                                                      |                                                                                                                                                                                                                                                                                                                            
|                                                      | Nine Months EndedSeptember 30,                                                                                                                                                                                                                                                                                             
|                                                      | 2012                                                                                                                                                                                                                                                                                                                       
|(millions)                                            |                                                                                                                                                                                                                                                                                                                            
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                          
|Liabilities Incurred                                  | 24                                                                                                                                                                                                                                                                                                                         
|Liabilities Settled                                   | (98                                                                                                                                                                                                                                                                                                                        
|Revision of Estimate                                  | 30                                                                                                                                                                                                                                                                                                                         
|Accretion Expense                                     | 21                                                                                                                                                                                                                                                                                                                         
|Other                                                 | (34                                                                                                                                                                                                                                                                                                                        
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                          

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