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Revenue Customer Type (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
USD ($)
Units
Dec. 31, 2020
USD ($)
Sep. 30, 2023
USD ($)
Sep. 30, 2022
USD ($)
Revenue, Major Customer [Line Items]        
MLGW's % of operating revenues 800.00%      
NES's % of operating revenues 800.00%      
Sales of Electricity $ 2,538 $ 2,270    
Electric revenue 2,537 2,268    
Off System Sales of Electricity 1      
Other revenue 45 34    
Revenues 2,583 2,304    
Bill credits for LTA $ (43) (42)    
Number of LPCs signed LTA | Units 146      
Number of LPCs signed Flexibility Agreement | Units 76      
Percent of sales of electricity to LPCs 91.00%      
Pandemic Relief Credit $ (50) (49)    
Forecast        
Revenue, Major Customer [Line Items]        
Pandemic Relief Credit     $ 133 $ 220
lpcs [Domain]        
Revenue, Major Customer [Line Items]        
Electric revenue 2,306 2,091    
industries directly served [Domain]        
Revenue, Major Customer [Line Items]        
Electric revenue 205 154    
federal agencies and other [Domain]        
Revenue, Major Customer [Line Items]        
Electric revenue 27 25    
ALABAMA        
Revenue, Major Customer [Line Items]        
Electric revenue 371 334    
GEORGIA        
Revenue, Major Customer [Line Items]        
Electric revenue 64 58    
KENTUCKY        
Revenue, Major Customer [Line Items]        
Electric revenue 171 137    
MISSISSIPPI        
Revenue, Major Customer [Line Items]        
Electric revenue 241 212    
NORTH CAROLINA        
Revenue, Major Customer [Line Items]        
Electric revenue 20 16    
TENNESSEE        
Revenue, Major Customer [Line Items]        
Electric revenue 1,659 1,501    
VIRGINIA        
Revenue, Major Customer [Line Items]        
Electric revenue $ 11 $ 10