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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)
 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Interest rate swaps $ 840,000,000 $ 662,000,000  
Ineffective portion excluded from testing 0    
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred 1,000,000    
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax 48,000,000 42,000,000 $ (93,000,000)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax $ 25,000,000 $ 99,000,000 $ (157,000,000)