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Derivatives - Schedule of Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - Long-term debt - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Derivative [Line Items] |  |
Carrying Amount of the Hedged Item | $ (3,733) | $ (2,936)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | 164 | 242
SOUTHERN Co GAS |  |
Derivative [Line Items] |  |
Carrying Amount of the Hedged Item | (445) | (422)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | $ 52 | $ 75

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