| **Asset Impairments (Tables)**                                                       | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
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| **Impairments [Abstract]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Details of Impairment of Long-Lived Assets Held and Used by Asset [Table Text Block] | Pre-tax (non-cash) asset impairment charges were as follows: Three Months EndedSeptember 30, Nine Months EndedSeptember 30, 2012 2011 2012 2011(millions)       South Raton (Deepwater Gulf of Mexico)$— $— $34 $—Piceance (Onshore US)— — 39 —East Texas (Onshore US)— — — 116Other (Onshore US)— — — 21Total$— $— $73 $137 |

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| - Definition
Tabular disclosure for impairment of long-lived assets held and used by an entity which includes a description of the impaired long-lived asset and facts and circumstances leading to the impairment, aggregate amount of the impairment loss and where the loss is located in the income statement, method(s) for determining fair value, and the segment in which the impaired long-lived asset is reported.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 144 -Paragraph 26 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section CC -Subsection 3
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