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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Sep. 30, 2023
State [Line Items]          
Electric revenue $ (2,662) $ (2,925) $ (8,542) $ (8,306)  
Sales of Electricity 2,664 2,932 8,551 8,318  
Off System Sales of Electricity 2   9 12  
Other revenue $ 34 38 $ 121 119  
Total long duration contract revenue recognition 2,431   7,811    
Revenues $ 2,698 2,970 $ 8,672 8,437  
Off system sales   7      
Percent of wholesale Credit offered     310.00%    
Percent of Pandemic Credit Offered     2.50%    
Pandemic Relief Credit (52) (56) $ (158) (161)  
Percent of Generation Flexibility Credit     5.00%    
NES's % of operating revenues     800.00%    
Percent of Generation Flexibility Credit     5.00%    
Forecast          
State [Line Items]          
Pandemic Relief Credit         $ 230
ALABAMA          
State [Line Items]          
Electric revenue (388) (419) $ (1,244) (1,203)  
GEORGIA          
State [Line Items]          
Electric revenue (59) (69) (207) (207)  
KENTUCKY          
State [Line Items]          
Electric revenue (172) (195) (562) (556)  
MISSISSIPPI          
State [Line Items]          
Electric revenue (263) (283) (814) (788)  
NORTH CAROLINA          
State [Line Items]          
Electric revenue (17) (20) (67) (63)  
TENNESSEE          
State [Line Items]          
Electric revenue (1,754) (1,929) (5,613) (5,455)  
VIRGINIA          
State [Line Items]          
Electric revenue (9) $ (10) (35) $ (34)  
20-year Termination Notice [Member]          
State [Line Items]          
Revenues 2,073   6,741    
5-year termination notice [Member]          
State [Line Items]          
Revenues $ 358   $ 1,070