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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, 2024 | Dec. 31, 2023
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Derivative [Line Items] |  |
Carrying Amount of the Hedged Item | $ (3,085) | $ (3,024)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | 193 | 235
SOUTHERN Co GAS |  |
Derivative [Line Items] |  |
Carrying Amount of the Hedged Item | (440) | (427)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | $ 57 | $ 70

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