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ACQUISITIONS AND DISPOSITIONS - Schedule of Pre-tax Income (Details) - Southern Power - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Plant Nacogdoches |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |
Earnings before income taxes |  | $ 13
Mankato |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |
Earnings before income taxes | $ 2 | $ 29

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