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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)
 12 Months Ended
Sep. 30, 2025
USD ($)
Sep. 30, 2024
USD ($)
Derivative    
Change in Unrealized gains (losses) on Interest Rate Derivatives $ (145,000,000) $ (182,000,000)
Fair value 12,000,000 64,000,000
Interest rate swaps 698,000,000 840,000,000
Interest Rate Swap    
Derivative    
Unrealized gains/losses on derivatives 45,000,000 $ (31,000,000)
Unrealized gains/losses on derivatives $ 0  
Commodity Contract Derivatives    
Derivative    
Number of contracts 53 45
Notional amount 562,000,000 321,000,000
Fair value $ 10,000,000 $ 2,000,000
Commodity Contract under FHP    
Derivative    
Unrealized gains/losses on derivatives $ (93,000,000) $ (295,000,000)
Number of contracts 295 126
Notional amount 300 230
Fair value $ (57,000,000) $ (161,000,000)
Commodity Contract under FHP | Fuel Expense    
Derivative    
Unrealized gains/losses on derivatives 77,000,000 245,000,000
Commodity Contract under FHP | Purchased Power Expense    
Derivative    
Unrealized gains/losses on derivatives 16,000,000 50,000,000
Accounts payable and accrued liabilities | Commodity Contract Derivatives    
Derivative    
Fair value (2,000,000) (3,000,000)
Accounts payable and accrued liabilities | Commodity Contract under FHP    
Derivative    
Fair value $ (57,000,000) $ (99,000,000)