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Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
State [Line Items]        
Electric revenue $ (2,531) $ (2,488) $ (4,799) $ (5,019)
Sales of Electricity 2,533 2,489 4,803 5,021
Off System Sales of Electricity 2   4 2
Other revenue $ 39 32 $ 73 78
Total long duration contract revenue recognition 2,337   4,428  
Revenues $ 2,572 2,521 $ 4,876 5,099
Off system sales   1    
Percent of wholesale Credit offered     310.00%  
Percent of Pandemic Credit Offered     250.00%  
ALABAMA        
State [Line Items]        
Electric revenue (379) (360) $ (713) (729)
GEORGIA        
State [Line Items]        
Electric revenue (66) (64) (124) (127)
KENTUCKY        
State [Line Items]        
Electric revenue (157) (158) (294) (315)
MISSISSIPPI        
State [Line Items]        
Electric revenue (237) (227) (449) (464)
NORTH CAROLINA        
State [Line Items]        
Electric revenue (18) (19) (34) (37)
TENNESSEE        
State [Line Items]        
Electric revenue (1,663) (1,648) (3,164) (3,324)
VIRGINIA        
State [Line Items]        
Electric revenue (11) $ (12) (21) $ (23)
20-year Termination Notice [Member]        
State [Line Items]        
Revenues 1,934   3,654  
5-year termination notice [Member]        
State [Line Items]        
Revenues $ 403   $ 774