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Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
$ / shares in Units, $ in Millions
Jun. 30, 2018
Dec. 31, 2017
Financial Liabilities Fair Value Disclosure [Abstract]    
Equity owned (units) 14,217,198 21,692,198
Tamar Petroleum Ltd    
Financial Liabilities Fair Value Disclosure [Abstract]    
Equity owned (units) 38,495,575  
Share price (in usd per share) $ 4.60  
Fair Value, Measurements, Recurring    
Financial Assets:    
Investments   $ 57
Commodity Derivative Instruments $ 29 2
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments (335) (73)
Portion of Deferred Compensation Liability Measured at Fair Value (73) (71)
Stock Based Compensation Liability Measured at Fair Value (12) (10)
Fair Value, Measurements, Recurring | 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 (73) (71)
Stock Based Compensation Liability Measured at Fair Value (12) (10)
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Financial Assets:    
Investments   0
Commodity Derivative Instruments 72 7
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments (378) (78)
Portion of Deferred Compensation Liability Measured at Fair Value 0 0
Stock Based Compensation Liability Measured at Fair Value 0 0
Fair Value, Measurements, Recurring | 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 0
Fair Value, Measurements, Recurring | Scenario, Adjustment    
Financial Assets:    
Investments   0
Commodity Derivative Instruments (43) (5)
Financial Liabilities Fair Value Disclosure [Abstract]    
Commodity Derivative Instruments 43 5
Portion of Deferred Compensation Liability Measured at Fair Value 0 0
Stock Based Compensation Liability Measured at Fair Value 0 $ 0
Mutual Fund Investments | Fair Value, Measurements, Recurring    
Financial Assets:    
Investments 57  
Mutual Fund Investments | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)    
Financial Assets:    
Investments 57  
Mutual Fund Investments | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Financial Assets:    
Investments 0  
Mutual Fund Investments | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Financial Assets:    
Investments 0  
Mutual Fund Investments | Fair Value, Measurements, Recurring | Scenario, Adjustment    
Financial Assets:    
Investments 0  
Investment in Tamar Petroleum Ltd. (38,495,575 Shares) (5) | Fair Value, Measurements, Recurring    
Financial Assets:    
Investments 150  
Investment in Tamar Petroleum Ltd. (38,495,575 Shares) (5) | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)    
Financial Assets:    
Investments 0  
Investment in Tamar Petroleum Ltd. (38,495,575 Shares) (5) | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Financial Assets:    
Investments 150  
Investment in Tamar Petroleum Ltd. (38,495,575 Shares) (5) | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Financial Assets:    
Investments 0  
Investment in Tamar Petroleum Ltd. (38,495,575 Shares) (5) | Fair Value, Measurements, Recurring | Scenario, Adjustment    
Financial Assets:    
Investments $ 0