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REGULATORY MATTERS - MPC Environmental Compliance Overview Plan (Details) - Mississippi Power - USD ($) $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Feb. 12, 2024 | Jun. 30, 2023 | Apr. 30, 2023 | Jun. 30, 2022 | Apr. 30, 2022 | Jun. 30, 2021 | May 31, 2021 | Dec. 31, 2021
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  | $ (7) |  | $ 18 |  | $ 16 | $ 28 | $ 9
Environmental Compliance Overview Plan |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  | $ 3 |  | $ 1 | $ (9) |  |
Environmental Compliance Overview Plan | Subsequent Event |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount | $ 9 |  |  |  |  |  |  |

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