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ASSET RETIREMENT OBLIGATIONS - Fair Value Increases (Decreases) of the Funds (Details) - Securities held in the Funds - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Fair value increases |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | $ 274 | $ 280 | $ 344
Unrealized gains (losses) |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | (27) | 220 | 259
Alabama Power | Fair value increases |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | 200 | 142 | 194
Alabama Power | Unrealized gains (losses) |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | (30) | 121 | 149
Georgia Power | Fair value increases |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | 74 | 138 | 150
Georgia Power | Unrealized gains (losses) |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Fair value increases (decreases) of the Funds | $ 3 | $ 99 | $ 110

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