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Derivatives - Schedule of Pre-tax Effects of Unrealized Derivative Gains (Losses) (Details) - Energy-related derivatives - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | $ 8 | $ (50)
Other regulatory assets, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (24) | (61)
Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (1) | (5)
Other regulatory liabilities, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 13 | 8
Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 20 | 8
ALABAMA POWER CO |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 6 | (16)
ALABAMA POWER CO | Other regulatory assets, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (7) | (23)
ALABAMA POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (1) | 0
ALABAMA POWER CO | Other regulatory liabilities, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 7 | 4
ALABAMA POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 7 | 3
GEORGIA POWER CO |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 1 | (22)
GEORGIA POWER CO | Other regulatory assets, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (8) | (26)
GEORGIA POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 0 | 0
GEORGIA POWER CO | Other regulatory liabilities, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 1 | 0
GEORGIA POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 8 | 4
MISSISSIPPI POWER CO |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (3) | (15)
MISSISSIPPI POWER CO | Other regulatory assets, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (8) | (11)
MISSISSIPPI POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 0 | (5)
MISSISSIPPI POWER CO | Other regulatory liabilities, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 0 | 0
MISSISSIPPI POWER CO | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 5 | 1
SOUTHERN Co GAS |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 4 | 3
SOUTHERN Co GAS | Other regulatory assets, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | (1) | (1)
SOUTHERN Co GAS | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 0 | 0
SOUTHERN Co GAS | Other regulatory liabilities, current |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | 5 | 4
SOUTHERN Co GAS | Other regulatory assets, deferred |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Total energy-related derivative gains (losses) | $ 0 | $ 0

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