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Revenue (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Sales Revenue, Net [Member] | Indiana [Member] |  |
Percentage of revenue | 73.00% | 68.00%
Fixed-price Contract [Member] |  |
Performance obligation | $ 840 |
Percentage of revenue expected to be recognized by set date | 68.00% |
Revenue, remaining performance obligation, expected timing of satisfaction, year | 2020 |
Revenue, remaining performance obligation, expected timing of satisfaction, explanation | We expect to recognize approximately 68% of this revenue through 2020, with the remainder recognized thereafter. |
Index Priced Contracts [Member] |  |
Performance obligation | $ 330 |
Revenue, remaining performance obligation, expected timing of satisfaction, year | 2020 |
Revenue, remaining performance obligation, expected timing of satisfaction, explanation | We expect to recognize all of this income beginning in 2020. |

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