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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Costs for Affiliate Transactions (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Alabama Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 611 | $ 549 | $ 504
Alabama Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 12 | 18 | 14
Georgia Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 857 | 762 | 663
Georgia Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 101 | 99 | 108
Mississippi Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 113 | 115 | 120
Mississippi Power | Alabama Power | Power Pool |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 4 | 4 | 9
Southern Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 86 | 86 | 89
Southern Power | Alabama Power | Power Pool |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 13 | 29 | 15
Southern Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 34 | 37 | 31
Southern Company Gas | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 261 | 262 | 239
Southern Company Gas | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 28 | $ 27 | $ 29

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