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ASSET RETIREMENT OBLIGATIONS - Narrative (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Cash flow revisions |  |  | $ 1,026 | $ 912 |
Alabama Power |  |  |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Increase to AROs |  |  |  | 501 |
Cash flow revisions |  |  | $ 406 | 501 |
Estimated inflation rate |  |  | 4.50% |  |
Estimated trust earnings rate |  |  | 7.00% |  |
Alabama Power | Internal reserves |  |  |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Accumulated provisions for decommissioning |  |  | $ 15 | 15 |
Georgia Power |  |  |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Increase to AROs |  | $ 411 |  |  |
Cash flow revisions | $ 435 |  | 530 | 418 |
Fair market value of securities on loan and pledged to creditors |  |  | 42 | 44 |
Fair value of collateral received |  |  | $ 43 | 45 |
Estimated inflation rate |  |  | 2.50% |  |
Estimated trust earnings rate |  |  | 4.50% |  |
Georgia Power | Plant Hatch and Plant Vogtle Units 1 And 2 |  |  |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Annual decommissioning cost for ratemaking |  |  |  | 4 | $ 5
Mississippi Power |  |  |  |  |
Jointly Owned Utility Plant Interests [Line Items] |  |  |  |  |
Cash flow revisions | $ 31 |  | $ 31 | $ 0 |

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