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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Mar. 31, 2021
Mar. 31, 2022
Mar. 31, 2021
Sep. 30, 2023
Sep. 30, 2022
State [Line Items]              
Electric revenue $ (2,844)   $ (2,531) $ (5,381) $ (4,799)    
Sales of Electricity 2,848   2,533 5,386 4,803    
Off System Sales of Electricity 4     5 4    
Other revenue $ 36   39 $ 81 73    
Total long duration contract revenue recognition 2,608     4,914      
Revenues $ 2,884   2,572 $ 5,467 4,876    
Off system sales     2        
Percent of wholesale Credit offered       310.00%      
Percent of Pandemic Credit Offered   2.50%   2.50%      
Pandemic Relief Credit (55)     $ (105) (104)    
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]              
Percent of Pandemic Credit Offered           1.50%  
Pandemic Relief Credit           $ 133 $ 220
ALABAMA              
State [Line Items]              
Electric revenue (413)   (379) $ (784) (713)    
GEORGIA              
State [Line Items]              
Electric revenue (74)   (66) (138) (124)    
KENTUCKY              
State [Line Items]              
Electric revenue (190)   (157) (361) (294)    
MISSISSIPPI              
State [Line Items]              
Electric revenue (264)   (237) (505) (449)    
NORTH CAROLINA              
State [Line Items]              
Electric revenue (23)   (18) (43) (34)    
TENNESSEE              
State [Line Items]              
Electric revenue (1,867)   (1,663) (3,526) (3,164)    
VIRGINIA              
State [Line Items]              
Electric revenue (13)   $ (11) (24) $ (21)    
20-year Termination Notice [Member]              
State [Line Items]              
Revenues 2,250     4,223      
5-year termination notice [Member]              
State [Line Items]              
Revenues $ 358     $ 691