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REGULATORY MATTERS - GAS Natural Gas Cost Recovery (Details) - Southern Company Gas - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Public Utilities, General Disclosures [Line Items] |  |
Over recovered fuel cost | $ (473) | $ 88
Natural Gas Cost Under Recovery |  |
Public Utilities, General Disclosures [Line Items] |  |
Over recovered fuel cost | (266) |
Other regulatory assets, deferred |  |
Public Utilities, General Disclosures [Line Items] |  |
Over recovered fuel cost | $ (207) |

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