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DERIVATIVES - Narrative (Details) MMBTU in Millions | 12 Months Ended
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Dec. 31, 2023 USD ($) MMBTU | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Derivative [Line Items] |  |  |
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU | 12 |  |
Estimated pre-tax gains (losses) expected to be reclassified from AOCI | $ (37,000,000) |  |
Estimated pre-tax gains (losses) that will be reclassified from OCI to interest expense for the next 12-month period | 19,000,000 |  |
Collateral already posted, aggregate fair value | 62,000,000 |  |
Derivative liability | 659,000,000 | $ 617,000,000 |
Interest rate derivatives |  |  |
Derivative [Line Items] |  |  |
Derivative liability | 52,000,000 |  |
Interest rate derivatives | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | (12,000,000) | 46,000,000 | $ 5,000,000
Energy-related derivatives | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | $ (81,000,000) | 3,000,000 | 34,000,000
Alabama Power |  |  |
Derivative [Line Items] |  |  |
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU | 3 |  |
Collateral already posted, aggregate fair value | $ 0 |  |
Derivative liability | 100,000,000 | 15,000,000 |
Alabama Power | Energy-related derivatives | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | $ 0 |  |
Georgia Power |  |  |
Derivative [Line Items] |  |  |
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU | 4 |  |
Derivative liability | $ 113,000,000 | 41,000,000 |
Georgia Power | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | $ (2,000,000) | 31,000,000 | 0
Mississippi Power |  |  |
Derivative [Line Items] |  |  |
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU | 2 |  |
Derivative liability | $ 47,000,000 | 15,000,000 |
Southern Power |  |  |
Derivative [Line Items] |  |  |
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU | 3 |  |
Estimated pre-tax gains (losses) expected to be reclassified from AOCI | $ (4,000,000) |  |
Estimated pre-tax gains (losses) that will be reclassified from OCI to interest expense for the next 12-month period | 11,000,000 |  |
Collateral already posted, aggregate fair value | 0 |  |
Derivative liability | 27,000,000 | 59,000,000 |
Southern Power | Interest rate derivatives |  |  |
Derivative [Line Items] |  |  |
Derivative liability | 0 |  |
Southern Power | Energy-related derivatives | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | (18,000,000) | (15,000,000) | 12,000,000
Registrants | Derivative Counterparties |  |  |
Derivative [Line Items] |  |  |
Collateral already posted, aggregate fair value | 0 |  |
Southern Company Gas |  |  |
Derivative [Line Items] |  |  |
Estimated pre-tax gains (losses) expected to be reclassified from AOCI | (33,000,000) |  |
Collateral already posted, aggregate fair value | 62,000,000 | 41,000,000 |
Derivative liability | 87,000,000 | 110,000,000 |
Southern Company Gas | Energy-related derivatives | Cash Flow Hedging |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) From Derivatives Recognized in OCI | $ (63,000,000) | $ 18,000,000 | $ 22,000,000

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