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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, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Alabama Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 504 | $ 478 | $ 527
Alabama Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 14 | 15 | 17
Georgia Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 663 | 639 | 704
Georgia Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 108 | 108 | 99
Mississippi Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 120 | 149 | 118
Mississippi Power | Alabama Power | Power Pool |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 9 | 4 | 3
Southern Power | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 89 | 87 | 90
Southern Power | Alabama Power | Power Pool |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 15 | 8 | 14
Southern Power | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 31 | 29 | 28
Southern Company Gas | Southern Company Services, Inc. |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | 239 | 237 | 183
Southern Company Gas | Southern Natural Gas Company, LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Costs for services | $ 29 | $ 29 | $ 31

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