|**Asset Retirement Obligations (Tables)**             | 12 Months Ended                                                                                                                                                                                                                                                                                                          
|----------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                          
|Schedule of Change in Asset Retirement Obligation     | Changes in asset retirement obligations were as follows:Year Ended December 31,(millions)20142013Asset Retirement Obligations, Beginning Balance$586$402Liabilities Incurred7590Liabilities Settled(101)(41)Revision of Estimate155156Accretion Expense3628Other—(49)Asset Retirement Obligations, Ending Balance$751$586
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|                                                      |                                                                                                                                                                                                                                                                                                                          
|                                                      | Year Ended December 31,                                                                                                                                                                                                                                                                                                  
|(millions)                                            | 2014                                                                                                                                                                                                                                                                                                                     
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                        
|Liabilities Incurred                                  | 75                                                                                                                                                                                                                                                                                                                       
|Liabilities Settled                                   | (101                                                                                                                                                                                                                                                                                                                     
|Revision of Estimate                                  | 155                                                                                                                                                                                                                                                                                                                      
|Accretion Expense                                     | 36                                                                                                                                                                                                                                                                                                                       
|Other                                                 | —                                                                                                                                                                                                                                                                                                                        
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                        

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