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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2020
Dec. 31, 2019
State [Line Items]    
Electric revenue $ (2,268) $ (2,531)
Sales of Electricity 2,270 2,532
Off System Sales of Electricity 2  
Other revenue $ 34 46
Total long duration contract revenue recognition 2,091  
Revenues $ 2,304 2,578
Off system sales   1
ALABAMA    
State [Line Items]    
Electric revenue (334) (369)
GEORGIA    
State [Line Items]    
Electric revenue (58) (63)
KENTUCKY    
State [Line Items]    
Electric revenue (137) (157)
MISSISSIPPI    
State [Line Items]    
Electric revenue (212) (237)
NORTH CAROLINA    
State [Line Items]    
Electric revenue (16) (18)
TENNESSEE    
State [Line Items]    
Electric revenue (1,501) (1,676)
VIRGINIA    
State [Line Items]    
Electric revenue (10) $ (11)
20-year Termination Notice [Member]    
State [Line Items]    
Revenues 1,720  
5-year termination notice [Member]    
State [Line Items]    
Revenues $ 371