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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Other Intangible Assets and Liabilities (Narrative) (Details) - USD ($) | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items] |  |  |
Goodwill impairment | $ 0 |  |
Goodwill | 5,280,000,000 | $ 5,280,000,000 | $ 5,280,000,000
Intangible liabilities included in other deferred credits and liabilities | 816,000,000 | 696,000,000 |
Southern Company Gas |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Goodwill impairment | 0 |  |
Goodwill | 5,015,000,000 | 5,015,000,000 |
Intangible liabilities included in other deferred credits and liabilities | $ 77,000,000 | $ 54,000,000 |
Southern Company Gas | Intangible liabilities |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Intangible liabilities included in other deferred credits and liabilities |  |  | $ 91,000,000

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 10 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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