|**Asset Retirement Obligations (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                     
|----------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                    
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Three Months EndedMarch 31,(millions)20162015Asset Retirement Obligations, Beginning Balance$989$751Liabilities Incurred210Liabilities Settled(8)(8)Revision of Estimate524Accretion Expense(1)1210Asset Retirement Obligations, Ending Balance$1,000$787(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
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|                                                      |                                                                                                                                                                                                                                                                                                                                                                                    
|                                                      | Three Months EndedMarch 31,                                                                                                                                                                                                                                                                                                                                                        
|(millions)                                            | 2016                                                                                                                                                                                                                                                                                                                                                                               
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                  
|Liabilities Incurred                                  | 2                                                                                                                                                                                                                                                                                                                                                                                  
|Liabilities Settled                                   | (8                                                                                                                                                                                                                                                                                                                                                                                 
|Revision of Estimate                                  | 5                                                                                                                                                                                                                                                                                                                                                                                  
|Accretion Expense(1)                                  | 12                                                                                                                                                                                                                                                                                                                                                                                 
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                  

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+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859
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