|**Asset Impairments (Notes)**          | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|**Impairments [Abstract]**             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Asset Impairment Charges [Text Block]  | Asset ImpairmentsPre-tax (non-cash) asset impairment charges were as follows:Three Months EndedSeptember 30,Nine Months EndedSeptember 30,2012201120122011(millions)South Raton (Deepwater Gulf of Mexico)$—$—$34$—Piceance (Onshore US)——39—East Texas (Onshore US)———116Other (Onshore US)———21Total$—$—$73$1372012During the second quarter of 2012,  we determined that the carrying amounts of our South Raton and Piceance developments were not recoverable from future cash flows and were impaired. The South Raton and Piceance impairments were primarily due to declines in near-term crude oil and natural gas prices, respectively. The assets were written down to their estimated fair values, which were determined using discounted cash flow models.2011Due to field performance combined with a low natural gas price environment, we determined that the carrying amounts of certain of our onshore US developments, primarily in East Texas, were not recoverable from future cash flows and, therefore, were impaired. The assets were written down to their estimated fair values, which were determined using discounted cash flow models.See Note 7. Fair Value Measurements and Disclosures.
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|                                       | Three Months EndedSeptember 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|                                       | 2012                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|(millions)                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|South Raton (Deepwater Gulf of Mexico) | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Piceance (Onshore US)                  | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|East Texas (Onshore US)                | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Other (Onshore US)                     | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Total                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

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