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Revenue Customer Type (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2022
USD ($)
Units
Dec. 31, 2021
USD ($)
Sep. 30, 2023
USD ($)
Revenue, Major Customer [Line Items]      
MLGW's % of operating revenues 800.00%    
NES's % of operating revenues 800.00%    
Sales of Electricity $ 2,963 $ 2,538  
Electric revenue 2,959 2,537  
Off System Sales of Electricity 4    
Other revenue 52 45  
Revenues 3,015 2,583  
Bill credits for LTA $ (48) (43)  
Number of LPCs signed LTA | Units 147    
Number of LPCs signed Flexibility Agreement | Units 81    
Percent of sales of electricity to LPCs 91.00%    
Pandemic Relief Credit $ (53) (50)  
Forecast      
Revenue, Major Customer [Line Items]      
Pandemic Relief Credit     $ 230
lpcs [Domain]      
Revenue, Major Customer [Line Items]      
Electric revenue 2,706 2,306  
industries directly served [Domain]      
Revenue, Major Customer [Line Items]      
Electric revenue 225 205  
federal agencies and other [Domain]      
Revenue, Major Customer [Line Items]      
Electric revenue 32 27  
ALABAMA      
Revenue, Major Customer [Line Items]      
Electric revenue 428 371  
GEORGIA      
Revenue, Major Customer [Line Items]      
Electric revenue 76 64  
KENTUCKY      
Revenue, Major Customer [Line Items]      
Electric revenue 198 171  
MISSISSIPPI      
Revenue, Major Customer [Line Items]      
Electric revenue 278 241  
NORTH CAROLINA      
Revenue, Major Customer [Line Items]      
Electric revenue 25 20  
TENNESSEE      
Revenue, Major Customer [Line Items]      
Electric revenue 1,941 1,659  
VIRGINIA      
Revenue, Major Customer [Line Items]      
Electric revenue $ 13 $ 11