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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Oct. 02, 2024
Sep. 30, 2013
Debt Instrument          
Face Amount $ 40       $ 360
Discount on debt issues (33) $ (9)      
Redemptions/Maturities of variable interest entities 41 35 $ 39    
Redemptions/Maturities of power bonds 1,022 1,022 $ 29    
Total Current maturities of power bonds issued at par 1,370 1,022      
Short-term debt, net of discounts 0 1,167      
Current maturities of long-term debt of variable interest entities issued at par 49 37      
Variable Interest Entity, Primary Beneficiary [Member]          
Debt Instrument          
Debt and Lease Obligation   0   $ 800  
880591EF5 (12.15.20)          
Debt Instrument          
Total Current maturities of power bonds issued at par $ 20 1      
Interest rate 3.77%        
Debt Instrument, Maturity Date Dec. 15, 2025        
880591EF5 (6.15.21)          
Debt Instrument          
Total Current maturities of power bonds issued at par $ 0 21      
Interest rate 3.77%        
Debt Instrument, Maturity Date Jun. 15, 2026        
880591EW8 [Member]          
Debt Instrument          
Total Current maturities of power bonds issued at par $ 0 1,000      
Interest rate 0.75%        
Debt Instrument, Maturity Date May 15, 2025        
880591CJ9          
Debt Instrument          
Total Current maturities of power bonds issued at par $ 1,350 $ 0      
Interest rate 6.75%        
Debt Instrument, Maturity Date Nov. 01, 2025        
Total          
Debt Instrument          
Debt Instrument, Redemption Period, End Date 1,063 1,057      
Percent of par value 100.00%        
Debt of variable interest entities          
Debt Instrument          
Redemptions/Maturities of variable interest entities $ 41 $ 35      
2009 Series B          
Debt Instrument          
Redemptions/Maturities of power bonds 22 22      
2012 Series A          
Debt Instrument          
Redemptions/Maturities of power bonds 0 1,000      
2020 Series A          
Debt Instrument          
Redemptions/Maturities of power bonds 1,000 0      
Total          
Debt Instrument          
Debt Securities Issues $ 4,767 991      
880591EY4          
Debt Instrument          
Interest rate 4.25%        
Debt Instrument, Maturity Date Sep. 15, 2052        
880591EX6          
Debt Instrument          
Interest rate 1.50%        
Debt Instrument, Maturity Date Sep. 15, 2031        
880591EZ1          
Debt Instrument          
Interest rate 3.875%        
Debt Instrument, Maturity Date Mar. 15, 2028        
880591FB3          
Debt Instrument          
Percent of par value 99.109%        
Interest rate 4.375%        
Debt Instrument, Maturity Date Aug. 01, 2034        
Debt Instrument, Issued, Principal $ 0 1,000      
880591FC1          
Debt Instrument          
Percent of par value 98.517%        
Interest rate 5.25%        
Debt Instrument, Maturity Date Feb. 01, 2055        
Debt Instrument, Issued, Principal $ 1,250 0      
880591FD9          
Debt Instrument          
Percent of par value 99.36%        
Interest rate 4.875%        
Debt Instrument, Maturity Date May 15, 2035        
Debt Instrument, Issued, Principal $ 1,500 0      
880591FE7          
Debt Instrument          
Percent of par value 99.593%        
Interest rate 3.875%        
Debt Instrument, Maturity Date Aug. 01, 2030        
Debt Instrument, Issued, Principal $ 1,250 $ 0