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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Southern Nuclear Operating Company, Inc. | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 251 | $ 267 | $ 258
Southern Nuclear Operating Company, Inc. | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 899 | 895 | 906
Southern Power | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 143 | 151 | 132
Southern Natural Gas Company, LLC | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 12 | 18 | 14
Southern Natural Gas Company, LLC | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 101 | 99 | 108
Southern Natural Gas Company, LLC | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 34 | 37 | 31
Southern Natural Gas Company, LLC | Southern Company Gas |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 28 | 27 | 29
Southern Company Gas | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 0 | 0 | 18
Southern Company Services, Inc. | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 611 | 549 | 504
Southern Company Services, Inc. | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 857 | 762 | 663
Southern Company Services, Inc. | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 86 | 86 | 89
Southern Company Services, Inc. | Southern Company Gas |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 261 | 262 | 239
PowerSecure | Southern Company Gas | US General Services Administration |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 29 | 10 |
Contract amount |  | 77 |
Non-Fuel Expense | Mississippi Power | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 5 | 6 | 10
Purchased Power from Affiliates | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 145 | 154 | 139
Operating Lease PPA | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 116 | 116 | 112
Electric Transmission | Southern Company Services, Inc. | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 33 | $ 39 | $ 28

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