|**Asset Retirement Obligations (Tables)**             | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                           
|----------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                          
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Six Months EndedJune 30,20132012(millions)Asset Retirement Obligations, Beginning Balance$402$377Liabilities Incurred223Liabilities Settled(10)(2)Revision of Estimate720Accretion Expense(1)1414Other—(89)Asset Retirement Obligations, Ending Balance$415$343(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
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|                                                      |                                                                                                                                                                                                                                                                                                                                                                                          
|                                                      | Six Months EndedJune 30,                                                                                                                                                                                                                                                                                                                                                                 
|                                                      | 2013                                                                                                                                                                                                                                                                                                                                                                                     
|(millions)                                            |                                                                                                                                                                                                                                                                                                                                                                                          
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                        
|Liabilities Incurred                                  | 2                                                                                                                                                                                                                                                                                                                                                                                        
|Liabilities Settled                                   | (10                                                                                                                                                                                                                                                                                                                                                                                      
|Revision of Estimate                                  | 7                                                                                                                                                                                                                                                                                                                                                                                        
|Accretion Expense(1)                                  | 14                                                                                                                                                                                                                                                                                                                                                                                       
|Other                                                 | —                                                                                                                                                                                                                                                                                                                                                                                        
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                        

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