|**Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Nonrecurring Basis (Details 2) (USD $)In Millions, unless otherwise specified**                                                                                                                   | 3 Months Ended                                                                                                                                                                                                                                                    | 6 Months Ended                                                                                                                                                                                                                                                   
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Asset Impairment Charges [Abstract]**                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|Impairment of Oil and Gas Properties                                                                                                                                                                                                                                                | $ 0                                                                                                                                                                                                                                                               | $ 73                                                                                                                                                                                                                                                             
|Discount Rate for Impairment Model                                                                                                                                                                                                                                                  | 10.00%                                                                                                                                                                                                                                                            | 10.00%                                                                                                                                                                                                                                                           
|Quoted Prices in Active Markets (Level 1)                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|**Asset Impairment Charges [Abstract]**                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|Impairment of Oil and Gas Properties                                                                                                                                                                                                                                                | 0                                                                                                                                                                                                                                                                 | 0                                                                                                                                                                                                                                                                
|Significant Other Observable Inputs (Level 2)                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|**Asset Impairment Charges [Abstract]**                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|Impairment of Oil and Gas Properties                                                                                                                                                                                                                                                | 0                                                                                                                                                                                                                                                                 | 0                                                                                                                                                                                                                                                                
|Significant Unobservable Inputs (Level 3)                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|**Asset Impairment Charges [Abstract]**                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|Impairment of Oil and Gas Properties                                                                                                                                                                                                                                                | 0                                                                                                                                                                                                                                                                 | 172                                                                                                                                                                                                                                                              
|Net Book Value                                                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|**Asset Impairment Charges [Abstract]**                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                  
|Impairment of Oil and Gas Properties                                                                                                                                                                                                                                                | $ 0                                                                                                                                                                                                                                                               | $ 245                                                                                                                                                                                                                                                            
|                                                                                                                                                                                                                                                                                   
|[1]
              Amounts for 2012 related primarily to our South Raton development in the deepwater Gulf of Mexico and our Piceance development, onshore US. The assets were written down to their estimated fair values, which were determined using discounted cash flow models. | [1]                                                                                                                                                                                                                                                               | Amounts for 2012 related primarily to our South Raton development in the deepwater Gulf of Mexico and our Piceance development, onshore US. The assets were written down to their estimated fair values, which were determined using discounted cash flow models.
|[1]                                                                                                                                                                                                                                                                                 | Amounts for 2012 related primarily to our South Raton development in the deepwater Gulf of Mexico and our Piceance development, onshore US. The assets were written down to their estimated fair values, which were determined using discounted cash flow models.

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