|**Asset Retirement Obligations (Tables)**             | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                           
|----------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                          
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Nine Months EndedSeptember 30,(millions)20152014Asset Retirement Obligations, Beginning Balance$751$586Liabilities Incurred5438Liabilities Settled(29)(77)Revision of Estimate79123Accretion Expense(1)3228Asset Retirement Obligations, Ending Balance$887$698(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
|                                                     
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                          
|                                                      | Nine Months EndedSeptember 30,                                                                                                                                                                                                                                                                                                                                                           
|(millions)                                            | 2015                                                                                                                                                                                                                                                                                                                                                                                     
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                        
|Liabilities Incurred                                  | 54                                                                                                                                                                                                                                                                                                                                                                                       
|Liabilities Settled                                   | (29                                                                                                                                                                                                                                                                                                                                                                                      
|Revision of Estimate                                  | 79                                                                                                                                                                                                                                                                                                                                                                                       
|Accretion Expense(1)                                  | 32                                                                                                                                                                                                                                                                                                                                                                                       
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                        

X- ReferencesNo definition available.
+ Details**Name:**
us-gaap_AssetRetirementObligationDisclosureAbstract**Namespace Prefix:**
us-gaap_**Data Type:**
xbrli:stringItemType**Balance Type:**
na**Period Type:**
duration X- DefinitionTabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation.
+ 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
+ Details**Name:**
us-gaap_ScheduleOfChangeInAssetRetirementObligationTableTextBlock**Namespace Prefix:**
us-gaap_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration