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REGULATORY MATTERS - APC Certificate of Convenience and Necessity and RGC (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 23, 2021 USD ($) MW | Mar. 31, 2024 agreement | Dec. 31, 2021 USD ($) project agreement MW | Dec. 31, 2020 USD ($) | Aug. 31, 2020 USD ($) MW | Dec. 31, 2019 MW
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
Construction work in progress | $ |  |  | $ 8,771 | $ 8,697 |  |
Alabama Power |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
Approved additional generating capacity, demand-side management and distributed energy resource programs |  |  |  |  | 200 |
Additional generating capacity, petitioned megawatts, total |  |  |  |  |  | 2,400
Construction work in progress | $ |  |  | $ 1,147 | $ 866 |  |
Renewable generation certificate, authorized generating capacity (in MWs) |  |  | 500 |  |  |
Renewable generation certificate, generating capacity (in MWs) |  |  | 250 |  |  |
Renewable generation certificate, number of projects approved | project |  |  | 5 |  |  |
Renewable generation certificate, number of projects | project |  |  | 2 |  |  |
Renewable generation certificate, generating capacity owned by parent (in MWs) |  |  | 18 |  |  |
Number of purchase power agreements | agreement |  |  | 3 |  |  |
Alabama Power | Scenario, Forecast |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
Number of purchase power agreements commencing in period | agreement |  | 2 |  |  |  |
Alabama Power | Calhoun Power Company, LLC |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
Approximate nameplate capacity (in MWs) | 743 |  |  |  |  |
Consideration transferred | $ | $ 180 |  |  |  |  |
Alabama Power | Year 2020 |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
PPA, additional generating capacity (in MWs) |  |  |  |  | 240 |
Alabama Power | Years 2015 |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
PPA, additional generating capacity (in MWs) |  |  | 400 |  |  |
Plant Barry Unit 8 | Alabama Power |  |  |  |  |  |
Schedule of Regulatory Assets and Liabilities [Line Items] |  |  |  |  |  |
Approximate nameplate capacity (in MWs) |  |  |  |  | 720 |
Costs incurred, percentage of estimated in-service cost |  |  |  |  | 5.00% |
Estimated in-service cost | $ |  |  |  |  | $ 652 |
Construction work in progress | $ |  |  | $ 304 |  |  |

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