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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, 2021
Sep. 30, 2020
Sep. 30, 2019
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Interest rate swaps $ 1,639,000,000 $ 2,041,000,000  
Ineffective portion excluded from testing 0    
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred 25,000,000    
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax 97,000,000 38,000,000 $ 45,000,000
Accounts payable and accrued liabilities      
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Interest rate swaps $ 115,000,000 $ 114,000,000