|**Divestitures (Tables)**                                                                 | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|----------------------------------------------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Acquisitions and Dispositions [Abstract]**                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Discontinued Operations Balance Sheet and Income Statement Disclosures [Table Text Block] | Summarized results of discontinued operations are as follows:Three Months Ended June 30,Six Months EndedJune 30,2013201220132012(millions)Oil and Gas Sales$12$65$21$140Income Before Income Taxes539479Income Tax Expense3221047Operating Income (Loss), Net of Tax217(6)32Gain on Sale, Net of Tax17—55—Discontinued Operations, Net of Tax$19$17$49$32                                                                                                                    
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|                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                                                          | Three Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|                                                                                          | 2013                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|(millions)                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Oil and Gas Sales                                                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Income Before Income Taxes                                                                | 5                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Income Tax Expense                                                                        | 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Operating Income (Loss), Net of Tax                                                       | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Gain on Sale, Net of Tax                                                                  | 17                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Discontinued Operations, Net of Tax                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Schedule of Disposal Groups, Gain on Asset Onshore Divestitures [Table Text Block]        | Onshore US PropertiesDuring the first six months of2013, we closed the sales of certain crude oil and natural gas properties in Kansas, Oklahoma and the Gulf Coast areas. The information regarding the assets sold is as follows:Six Months EndedJune 30,2013(millions)Sales Proceeds$60LessNet Book Value of Assets Sold(53)Goodwill Allocated to Assets Sold(4)Asset Retirement Obligations Associated with Assets Sold5Other Closing Adjustments4Gain on Divestitures$12
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|                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                                                          | Six Months EndedJune 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|                                                                                          | 2013                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|(millions)                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Sales Proceeds                                                                            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Less                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Net Book Value of Assets Sold                                                             | (53                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Goodwill Allocated to Assets Sold                                                         | (4                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Asset Retirement Obligations Associated with Assets Sold                                  | 5                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Other Closing Adjustments                                                                 | 4                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Gain on Divestitures                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

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