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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, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | $ 282 | $ 230 | $ 202
Alabama Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 68 | 61 | 71
Georgia Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 190 | 138 | 103
Mississippi Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 0 | 1 | 0
Southern Power |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | 6 | 11 | 15
Southern Company Gas |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Total AFUDC and interest capitalized | $ 18 | $ 18 | $ 13

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- Definition Amount capitalized of allowance for funds used during construction. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430 + Details
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