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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, 2024
USD ($)
Sep. 30, 2023
USD ($)
Derivative    
Change in Unrealized gains (losses) on Interest Rate Derivatives $ (182,000,000) $ (240,000,000)
Fair value 64,000,000 52,000,000
Interest rate swaps 840,000,000 662,000,000
Interest Rate Swap    
Derivative    
Unrealized gains/losses on derivatives (31,000,000) $ (45,000,000)
Unrealized gains/losses on derivatives $ 0  
Commodity Contract Derivatives    
Derivative    
Number of contracts 45 54
Notional amount 321,000,000 318,000,000
Fair value $ 2,000,000 $ 31,000,000
Commodity Contract under FHP    
Derivative    
Unrealized gains/losses on derivatives $ (295,000,000) $ (348,000,000)
Number of contracts 126 221
Notional amount 230 388
Fair value $ (161,000,000) $ (186,000,000)
Commodity Contract under FHP | Fuel Expense    
Derivative    
Unrealized gains/losses on derivatives (245,000,000) 301,000,000
Commodity Contract under FHP | Purchased Power Expense    
Derivative    
Unrealized gains/losses on derivatives (50,000,000) 47,000,000
Accounts payable and accrued liabilities | Commodity Contract Derivatives    
Derivative    
Fair value (3,000,000) (1,000,000)
Accounts payable and accrued liabilities | Commodity Contract under FHP    
Derivative    
Fair value $ (99,000,000) $ (135,000,000)