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Asset Sale and Exit Activities - Narrative (Details) - Retention Benefits - D.E. Karn Generating Complex $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Expected cost | $ 35 | $ 35 | $ 35
Retention costs | $ 4 | 11 | 14
Costs incurred and capitalized |  | $ 1 |
Property, Plant and Equipment |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Costs incurred and capitalized |  |  | $ 2

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