| **Acquisitions and Divestitures - Fair Value of Acquired Assets (Details) - USD ($) $ in Millions** | Dec. 31, 2018 | Sep. 30, 2018 | Dec. 31, 2017 | Apr. 24, 2017 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| **Business Acquisition [Line Items]**                                                               |          |          |          |          |
| Goodwill                                                                                            | $ 110         | $ 1,400       | $ 1,310       |          |
| Clayton Williams Energy                                                                             |          |          |          |          |
| **Business Acquisition [Line Items]**                                                               |          |          |          |          |
| Cash and Cash Equivalents                                                                           |          |          |          | $ 21          |
| Other Current Assets                                                                                |          |          |          | 70            |
| Proved Reserves                                                                                     |          |          |          | 722           |
| Undeveloped Leasehold Cost                                                                          |          |          |          | 1,571         |
| Gathering and Processing Assets                                                                     |          |          |          | 48            |
| Asset Retirement Costs                                                                              |          |          |          | 63            |
| Other Property Plant and Equipment                                                                  |          |          |          | 12            |
| Goodwill                                                                                            |          |          |          | 1,291         |
| Total Asset Value                                                                                   |          |          |          | $ 3,798       |

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
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