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Revenue Customer Type (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2024
USD ($)
Units
Dec. 31, 2023
USD ($)
Revenue, Major Customer [Line Items]    
MLGW's % of operating revenues 800.00% 800.00%
NES's % of operating revenues 800.00% 800.00%
Off System Sales of Electricity $ 1 $ 2
Bill credits for LTA $ (50) $ (47)
Number of LPCs signed LTA | Units 148  
Number of LPCs signed Flexibility Agreement | Units 102  
Percent of sales of electricity to LPCs 91.00% 91.00%
Pre-commercial Ops - capitalized revenue $ 0 $ 3
Regulated Operating Revenue 2,876 2,731
Unregulated Operating Revenue 44 34
Regulated and Unregulated Operating Revenue 2,920 2,765
lpcs [Domain]    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 2,616 2,487
industries directly served [Domain]    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 230 217
federal agencies and other [Domain]    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 30 30
ALABAMA    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 431 408
GEORGIA    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 74 69
KENTUCKY    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 182 177
MISSISSIPPI    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 269 255
NORTH CAROLINA    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 21 23
TENNESSEE    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue 1,886 1,789
VIRGINIA    
Revenue, Major Customer [Line Items]    
Regulated Operating Revenue $ 12 $ 11