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Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Financial Assets    
Investments   $ 57
Commodity Derivative Instruments $ 10 2
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments (132) (73)
Portion of Deferred Compensation Liability Measured at Fair Value (70) (71)
Stock Based Compensation Liability Measured at Fair Value   (10)
Quoted Prices in Active Markets (Level 1)    
Financial Assets    
Investments   57
Commodity Derivative Instruments 0 0
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments 0 0
Portion of Deferred Compensation Liability Measured at Fair Value (70) (71)
Stock Based Compensation Liability Measured at Fair Value   (10)
Significant Other Observable Inputs (Level 2)    
Financial Assets    
Investments   0
Commodity Derivative Instruments 29 7
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments (151) (78)
Portion of Deferred Compensation Liability Measured at Fair Value 0 0
Stock Based Compensation Liability Measured at Fair Value   0
Significant Unobservable Inputs (Level 3)    
Financial Assets    
Investments   0
Commodity Derivative Instruments 0 0
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments 0 0
Portion of Deferred Compensation Liability Measured at Fair Value 0 0
Stock Based Compensation Liability Measured at Fair Value   0
Scenario, Adjustment    
Financial Assets    
Investments   0
Commodity Derivative Instruments (19) (5)
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments 19 5
Portion of Deferred Compensation Liability Measured at Fair Value 0 0
Stock Based Compensation Liability Measured at Fair Value   $ 0
Mutual Fund Investments    
Financial Assets    
Investments 56  
Mutual Fund Investments | Quoted Prices in Active Markets (Level 1)    
Financial Assets    
Investments 56  
Mutual Fund Investments | Significant Other Observable Inputs (Level 2)    
Financial Assets    
Investments 0  
Mutual Fund Investments | Significant Unobservable Inputs (Level 3)    
Financial Assets    
Investments 0  
Mutual Fund Investments | Scenario, Adjustment    
Financial Assets    
Investments 0  
Investment in Tamar Petroleum Ltd.    
Financial Assets    
Investments 162  
Investment in Tamar Petroleum Ltd. | Quoted Prices in Active Markets (Level 1)    
Financial Assets    
Investments 0  
Investment in Tamar Petroleum Ltd. | Significant Other Observable Inputs (Level 2)    
Financial Assets    
Investments 162  
Investment in Tamar Petroleum Ltd. | Significant Unobservable Inputs (Level 3)    
Financial Assets    
Investments 0  
Investment in Tamar Petroleum Ltd. | Scenario, Adjustment    
Financial Assets    
Investments $ 0