|**Asset Retirement Obligations (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                     
|----------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                    
|Changes in Asset Retirement Obligations               | Changes in ARO are as follows:Three Months EndedMarch 31,(millions)20152014Asset Retirement Obligations, Beginning Balance$751$586Liabilities Incurred101Liabilities Settled(8)(14)Revision of Estimate2472Accretion Expense(1)1010Asset Retirement Obligations, Ending Balance$787$655(1)Accretion expense is included in DD&Aexpense in the consolidated statements ofoperations.
|                                                     
|                                                      |                                                                                                                                                                                                                                                                                                                                                                                    
|                                                      | Three Months EndedMarch 31,                                                                                                                                                                                                                                                                                                                                                        
|(millions)                                            | 2015                                                                                                                                                                                                                                                                                                                                                                               
|Asset Retirement Obligations, Beginning Balance       | $                                                                                                                                                                                                                                                                                                                                                                                  
|Liabilities Incurred                                  | 10                                                                                                                                                                                                                                                                                                                                                                                 
|Liabilities Settled                                   | (8                                                                                                                                                                                                                                                                                                                                                                                 
|Revision of Estimate                                  | 24                                                                                                                                                                                                                                                                                                                                                                                 
|Accretion Expense(1)                                  | 10                                                                                                                                                                                                                                                                                                                                                                                 
|Asset Retirement Obligations, Ending Balance          | $                                                                                                                                                                                                                                                                                                                                                                                  

X- 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