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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | $ 400 | $ 327 | $ 282
Alabama Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 109 | 90 | 68
Georgia Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 251 | 213 | 190
Southern Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 3 | 0 | 6
Southern Company Gas |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | $ 37 | $ 24 | $ 18

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