|**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 Ended March 31,(millions)20182017Asset Retirement Obligations, Beginning Balance$875$935Liabilities Incurred21Liabilities Settled(20)(9)Revisions of Estimates(11)(7)Reclassification to Liabilities Associated with Assets Held for Sale(227)—Accretion Expense(1)912Asset Retirement Obligations, Ending Balance$628$932(1)Accretion expense is included in depreciation, depletion and amortization (DD&A)expense in the consolidated statements ofoperations.
|                                                                    
|                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|                                                                     | Three Months Ended March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|(millions)                                                           | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Asset Retirement Obligations, Beginning Balance                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Liabilities Incurred                                                 | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Liabilities Settled                                                  | (20                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Revisions of Estimates                                               | (11                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Reclassification to Liabilities Associated with Assets Held for Sale | (227                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Accretion Expense(1)                                                 | 9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|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