| **Basis of Presentation - Performance Obligation, Expected Timing (Details) $ in Millions**               | Mar. 31, 2019 USD ($) |
|-----------------------------------------------------------------------------------------------------------|-----------------------|
| **Organization, Consolidation and Presentation of Financial Statements [Abstract]**                       |                  |
| Natural Gas Revenues                                                                                      | $ 224                 |
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01 |                  |
| **Organization, Consolidation and Presentation of Financial Statements [Abstract]**                       |                  |
| Expected timing                                                                                           | 9 months              |
| Natural Gas Revenues                                                                                      | $ 108                 |
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 |                  |
| **Organization, Consolidation and Presentation of Financial Statements [Abstract]**                       |                  |
| Expected timing                                                                                           | 1 year                |
| Natural Gas Revenues                                                                                      | $ 116                 |

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| - References
No definition available.
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| - Definition
Amount of transaction price allocated to performance obligation that has not been recognized as revenue.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045
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| - Definition
Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045
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