|**Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Recurring Basis (Details) (USD $)In Millions, unless otherwise specified**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | Sep. 30, 2013                                                                                                                                                                                                                                                          | Dec. 31, 2012                                                                                                                                                                                                                                                         
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|**Financial Assets [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Mutual Fund Investments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | $ 113                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 46                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                       
|**Financial Liabilities [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | (41)                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                       
|Portion of Deferred Compensation Liability Measured at Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | (190)                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                       
|Quoted Prices in Active Markets (Level 1)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|**Financial Assets [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Mutual Fund Investments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | 113                                                                                                                                                                                                                                                                    | [1]                                                                                                                                                                                                                                                                   
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 0                                                                                                                                                                                                                                                                      | [1]                                                                                                                                                                                                                                                                   
|**Financial Liabilities [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 0                                                                                                                                                                                                                                                                      | [1]                                                                                                                                                                                                                                                                   
|Portion of Deferred Compensation Liability Measured at Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | (190)                                                                                                                                                                                                                                                                  | [1]                                                                                                                                                                                                                                                                   
|Significant Other Observable Inputs (Level 2)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|**Financial Assets [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Mutual Fund Investments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | 0                                                                                                                                                                                                                                                                      | [2]                                                                                                                                                                                                                                                                   
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 72                                                                                                                                                                                                                                                                     | [2]                                                                                                                                                                                                                                                                   
|**Financial Liabilities [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | (67)                                                                                                                                                                                                                                                                   | [2]                                                                                                                                                                                                                                                                   
|Portion of Deferred Compensation Liability Measured at Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 0                                                                                                                                                                                                                                                                      | [2]                                                                                                                                                                                                                                                                   
|Significant Unobservable Inputs (Level 3)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|**Financial Assets [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Mutual Fund Investments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | 0                                                                                                                                                                                                                                                                      | [3]                                                                                                                                                                                                                                                                   
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 0                                                                                                                                                                                                                                                                      | [3]                                                                                                                                                                                                                                                                   
|**Financial Liabilities [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 0                                                                                                                                                                                                                                                                      | [3]                                                                                                                                                                                                                                                                   
|Portion of Deferred Compensation Liability Measured at Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 0                                                                                                                                                                                                                                                                      | [3]                                                                                                                                                                                                                                                                   
|Adjustment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|**Financial Assets [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Mutual Fund Investments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | 0                                                                                                                                                                                                                                                                      | [4]                                                                                                                                                                                                                                                                   
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | (26)                                                                                                                                                                                                                                                                   | [4]                                                                                                                                                                                                                                                                   
|**Financial Liabilities [Abstract]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                       
|Commodity Derivative Instruments                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | 26                                                                                                                                                                                                                                                                     | [4]                                                                                                                                                                                                                                                                   
|Portion of Deferred Compensation Liability Measured at Fair Value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | $ 0                                                                                                                                                                                                                                                                    | [4]                                                                                                                                                                                                                                                                   
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|[1]
              Level 1 measurements are fair value measurements which use quoted market prices (unadjusted) in active markets for identical assets or liabilities. We use Level 1 inputs when available as Level 1 inputs generally provide the most reliable evidence of fair value.[2]
              Level 2 measurements are fair value measurements which use inputs, other than quoted prices included within Level 1, which are observable for the asset or liability, either directly or indirectly.[3]
              Level 3 measurements are fair value measurements which use unobservable inputs.[4]
              Amount represents the impact of netting clauses within our master agreements that allow us to net cash settle asset and liability positions with the same counterparty. | [1]                                                                                                                                                                                                                                                                    | Level 1 measurements are fair value measurements which use quoted market prices (unadjusted) in active markets for identical assets or liabilities. We use Level 1 inputs when available as Level 1 inputs generally provide the most reliable evidence of fair value.
|[1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | Level 1 measurements are fair value measurements which use quoted market prices (unadjusted) in active markets for identical assets or liabilities. We use Level 1 inputs when available as Level 1 inputs generally provide the most reliable evidence of fair value.
|[2]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | Level 2 measurements are fair value measurements which use inputs, other than quoted prices included within Level 1, which are observable for the asset or liability, either directly or indirectly.                                                                  
|[3]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | Level 3 measurements are fair value measurements which use unobservable inputs.                                                                                                                                                                                       
|[4]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | Amount represents the impact of netting clauses within our master agreements that allow us to net cash settle asset and liability positions with the same counterparty.                                                                                               

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