| **Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions**         | Sep. 30, 2018 | Dec. 31, 2017 |
|----------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financial Liabilities Fair Value Disclosure [Abstract]**                                                                                   |          |          |
| Equity owned (units)                                                                                                                         | 0             | 21,692,198    |
| Tamar Petroleum Ltd                                                                                                                          |          |          |
| **Financial Liabilities Fair Value Disclosure [Abstract]**                                                                                   |          |          |
| Equity owned (units)                                                                                                                         | 38,495,575    |          |
| Fair Value, Measurements, Recurring                                                                                                          |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  |          | $ 57          |
| Commodity Derivative Instruments                                                                                                             | $ 0           | 2             |
| **Financial Liabilities Fair Value Disclosure [Abstract]**                                                                                   |          |          |
| Commodity Derivative Instruments                                                                                                             | (394)         | (73)          |
| Portion of Deferred Compensation Liability Measured at Fair Value                                                                            | (73)          | (71)          |
| Stock Based Compensation Liability Measured at Fair Value                                                                                    | (14)          | (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                                                                                    | (14)          | (10)          |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)                                                          |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  |          | 0             |
| Commodity Derivative Instruments                                                                                                             | 35            | 7             |
| **Financial Liabilities Fair Value Disclosure [Abstract]**                                                                                   |          |          |
| Commodity Derivative Instruments                                                                                                             | (429)         | (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                                                                                                             | (35)          | (5)           |
| **Financial Liabilities Fair Value Disclosure [Abstract]**                                                                                   |          |          |
| Commodity Derivative Instruments                                                                                                             | 35            | 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                                                                                                                                  | 58            |          |
| Mutual Fund Investments | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)                                    |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | 58            |          |
| 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) | Fair Value, Measurements, Recurring                                                 |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | 165           |          |
| Investment in Tamar Petroleum Ltd. (38,495,575 Shares) | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)     |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | 0             |          |
| Investment in Tamar Petroleum Ltd. (38,495,575 Shares) | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | 165           |          |
| Investment in Tamar Petroleum Ltd. (38,495,575 Shares) | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)     |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | 0             |          |
| Investment in Tamar Petroleum Ltd. (38,495,575 Shares) | Fair Value, Measurements, Recurring | Scenario, Adjustment                          |          |          |
| **Financial Assets:**                                                                                                                        |          |          |
| Investments                                                                                                                                  | $ 0           |          |

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| - Definition
This element represents a certain statement of financial position liability caption which represents a commodity derivative liability, measured at fair value on a recurring basis.
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This element represents a certain statement of financial position liability caption which represents a deferred compensation liability, measured at fair value on a recurring basis.
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| - Definition
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability
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| - Definition
Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.
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| - Definition
Balance held at close of period in number of shares.
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| - Definition
Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.
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