|**Asset Retirement Obligations (Tables)**             | 12 Months Ended                                                                                                                                                                                                                                                                                                
|----------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**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)20152014Asset Retirement Obligations, Beginning Balance$751$586Liabilities Incurred6775Liabilities Settled(38)(101)Revision of Estimate166155Accretion Expense4336Asset Retirement Obligations, Ending Balance$989$751
|                                                     
|                                                      |                                                                                                                                                                                                                                                                                                                
|                                                      | Year Ended December 31,                                                                                                                                                                                                                                                                                        
|(millions)                                            | 2015                                                                                                                                                                                                                                                                                                           
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                              
|Liabilities Incurred                                  | 67                                                                                                                                                                                                                                                                                                             
|Liabilities Settled                                   | (38                                                                                                                                                                                                                                                                                                            
|Revision of Estimate                                  | 166                                                                                                                                                                                                                                                                                                            
|Accretion Expense                                     | 43                                                                                                                                                                                                                                                                                                             
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                              

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