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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, 2023
USD ($)
Sep. 30, 2022
USD ($)
Derivative    
Change in Unrealized gains (losses) on Interest Rate Derivatives $ (240,000,000) $ (728,000,000)
Fair value 52,000,000 1,000,000
Interest rate swaps 662,000,000 $ 905,000,000
Unrealized gains/losses on derivatives $ 0  
Commodity Contract Derivatives    
Derivative    
Number of contracts 54 44
Notional amount 318,000,000 296,000,000
Fair value $ 31,000,000 $ 145,000,000
Commodity Contract under FHP    
Derivative    
Number of contracts 221 225
Notional amount 388 256
Fair value $ (186,000,000) $ 115,000,000
Commodity Contract under FHP | Fuel Expense    
Derivative    
Unrealized gains/losses on derivatives 301,000,000 38,000,000
Commodity Contract under FHP | Purchased Power Expense    
Derivative    
Unrealized gains/losses on derivatives 47,000,000 9,000,000
Accounts payable and accrued liabilities | Commodity Contract Derivatives    
Derivative    
Fair value (1,000,000) (6,000,000)
Accounts payable and accrued liabilities | Commodity Contract under FHP    
Derivative    
Fair value (135,000,000) (8,000,000)
Other Regulatory Assets (Liabilities) | Interest Rate Swap    
Derivative    
Unrealized gains/losses on derivatives (45,000,000) (103,000,000)
Other Regulatory Assets (Liabilities) | Commodity Contract under FHP    
Derivative    
Unrealized gains/losses on derivatives $ (348,000,000) $ 47,000,000