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Fair Value Measurements - Schedule of Increase (Decrease) In Fair Value Of Funds (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Increase (decrease) in fair value of funds | $ 132 | $ 97 | $ 290 | $ 230
ALABAMA POWER CO |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Increase (decrease) in fair value of funds | 75 | 67 | 160 | 153
GEORGIA POWER CO |  |  |  |
Fair Value, Option, Quantitative Disclosures [Line Items] |  |  |  |
Increase (decrease) in fair value of funds | $ 57 | $ 30 | $ 130 | $ 77

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