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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Mar. 31, 2023
Mar. 31, 2022
Sep. 30, 2023
State [Line Items]          
Electric revenue $ (2,921) $ (2,844) $ (5,880) $ (5,381)  
Sales of Electricity 2,924 2,848 5,887 5,386  
Off System Sales of Electricity 3   7 5  
Other revenue $ 35 36 $ 87 81  
Total long duration contract revenue recognition 2,674   5,380    
Revenues $ 2,959 2,884 $ 5,974 5,467  
Off system sales   4      
Percent of wholesale Credit offered     310.00%    
Percent of Pandemic Credit Offered     2.50%    
Pandemic Relief Credit (53) (55) $ (106) (105)  
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 (428) (413) $ (856) (784)  
GEORGIA          
State [Line Items]          
Electric revenue (72) (74) (148) (138)  
KENTUCKY          
State [Line Items]          
Electric revenue (192) (190) (390) (361)  
MISSISSIPPI          
State [Line Items]          
Electric revenue (273) (264) (551) (505)  
NORTH CAROLINA          
State [Line Items]          
Electric revenue (25) (23) (50) (43)  
TENNESSEE          
State [Line Items]          
Electric revenue (1,918) (1,867) (3,859) (3,526)  
VIRGINIA          
State [Line Items]          
Electric revenue (13) $ (13) (26) $ (24)  
20-year Termination Notice [Member]          
State [Line Items]          
Revenues 2,323   4,668    
5-year termination notice [Member]          
State [Line Items]          
Revenues $ 351   $ 712