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CONSOLIDATED ENTITIES AND EQUITY METHOD INVESTMENTS - Balance Sheet Information (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity investments in unconsolidated subsidiaries |  |  | $ 1,282 | $ 1,362 |
Impairment charges |  |  | 2 | 0 | $ 168
Southern Company Gas |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity investments in unconsolidated subsidiaries |  |  | 1,173 | 1,290 |
Impairment charges |  |  | 0 | 0 | $ 115
SNG | Southern Company Gas |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity investments in unconsolidated subsidiaries |  |  | 1,129 | 1,167 |
PennEast Pipeline | Southern Company Gas |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity investments in unconsolidated subsidiaries |  |  | 11 | 91 |
Impairment charges | $ 2 | $ 82 | 84 |  |
Other | Southern Company Gas |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity investments in unconsolidated subsidiaries |  |  | $ 33 | $ 32 |

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