| **Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs - Aging of Capitalized Exploratory Well Costs (Details) $ in Millions**          | Sep. 30, 2018 USD ($) project | Dec. 31, 2017 USD ($) project |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------|-------------------------------|
| **Extractive Industries [Abstract]**                                                                                                                  |                          |                          |
| Exploratory Well Costs Capitalized for a Period of One Year or Less                                                                                   | $ 8                           | $ 10                          |
| Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling                                                  | 350                           | 510                           |
| Balance at End of Period                                                                                                                              | $ 358                         | $ 520                         |
| Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling | project | 7                             | 8                             |

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| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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| - Definition
The amount of capitalized exploratory well costs that is pending the determination of proved reserves. This does not include amounts that were capitalized and subsequently expensed in the same annual period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of exploratory well costs that have been capitalized for a period of greater than one year after the completion of drilling.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of exploratory well costs that have been capitalized for a period of one year or less after the completion of drilling.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of projects for which exploratory well costs have been capitalized for a period of greater than one year after the completion of drilling.
+ References
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