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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
State [Line Items]          
Electric revenue $ (2,537) $ (2,268)      
Sales of Electricity 2,538 2,270      
Off System Sales of Electricity 1        
Other revenue $ 45 34      
Total long duration contract revenue recognition 2,306        
Revenues $ 2,583 2,304      
Off system sales   2      
Percent of wholesale Credit offered 310.00%        
Percent of Pandemic Credit Offered 2.50%       2.50%
Pandemic Relief Credit $ (50) (49)      
NES's % of operating revenues 800.00%        
Forecast          
State [Line Items]          
Percent of Pandemic Credit Offered     1.50%    
Pandemic Relief Credit     $ 133 $ 220  
ALABAMA          
State [Line Items]          
Electric revenue $ (371) (334)      
GEORGIA          
State [Line Items]          
Electric revenue (64) (58)      
KENTUCKY          
State [Line Items]          
Electric revenue (171) (137)      
MISSISSIPPI          
State [Line Items]          
Electric revenue (241) (212)      
NORTH CAROLINA          
State [Line Items]          
Electric revenue (20) (16)      
TENNESSEE          
State [Line Items]          
Electric revenue (1,659) (1,501)      
VIRGINIA          
State [Line Items]          
Electric revenue (11) $ (10)      
20-year Termination Notice [Member]          
State [Line Items]          
Revenues 1,973        
5-year termination notice [Member]          
State [Line Items]          
Revenues $ 333