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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Affiliate Transactions (Narrative) (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Southern Company Gas |  |  |
Related Party Transaction [Line Items] |  |  |
Proceeds from affiliate |  | $ 71,000,000 |
Southern Nuclear Operating Company, Inc. | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 258,000,000 | 262,000,000 | $ 256,000,000
Southern Nuclear Operating Company, Inc. | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 906,000,000 | 883,000,000 | 760,000,000
Southern Power | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 132,000,000 | 141,000,000 | 177,000,000
Southern Natural Gas Company, LLC | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 14,000,000 | 15,000,000 | 17,000,000
Southern Natural Gas Company, LLC | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 108,000,000 | 108,000,000 | 99,000,000
Proceeds from affiliate |  | 142,000,000 |
Southern Natural Gas Company, LLC | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 31,000,000 | 29,000,000 | 28,000,000
Southern Natural Gas Company, LLC | Southern Company Gas |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 29,000,000 | 29,000,000 | 31,000,000
Southern Company Gas | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 18,000,000 | 26,000,000 | 64,000,000
PowerSecure International, Inc. | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 0 |  |
PowerSecure International, Inc. | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services |  | 0 | 0
Southern Company Services, Inc. | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 504,000,000 | 478,000,000 | 527,000,000
Southern Company Services, Inc. | Georgia Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 663,000,000 | 639,000,000 | 704,000,000
Southern Company Services, Inc. | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 89,000,000 | 87,000,000 | 90,000,000
Southern Company Services, Inc. | Southern Company Gas |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 239,000,000 | 237,000,000 | 183,000,000
Non-Fuel Expense | Mississippi Power | Alabama Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 10,000,000 | 9,000,000 | 9,000,000
Purchased Power from Affiliates | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 139,000,000 | 139,000,000 | 174,000,000
Operating Lease PPA | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 112,000,000 | 115,000,000 | 116,000,000
Electric Transmission | Southern Company Services, Inc. | Southern Power |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 28,000,000 | $ 15,000,000 | $ 15,000,000

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