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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income and Other Taxes (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Southern Power |  |  |
Tax Credit Carryforward [Line Items] |  |  |
Percentage reduction in tax basis of assets | 50.00% |  |
Southern Company Gas |  |  |
Tax Credit Carryforward [Line Items] |  |  |
Excise taxes collected | $ 115 | $ 133 | $ 162
Included In Operating Expenses | Southern Company Gas |  |  |
Tax Credit Carryforward [Line Items] |  |  |
Excise taxes collected | $ 112 | $ 129 | $ 158

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