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REGULATORY MATTERS - GAS Natural Gas Cost Recovery (Details) - Southern Company Gas - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
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)