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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, 2020
Sep. 30, 2019
Sep. 30, 2018
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Interest rate swaps $ 2,041,000,000 $ 1,764,000,000  
Net unrealized gain (loss) on future cash flow hedges (1,000,000) (114,000,000) $ 10,000,000
Reclassification to earnings from cash flow hedges 38,000,000 45,000,000 $ 26,000,000
Ineffective portion excluded from testing 0    
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred 27,000,000    
Accounts payable and accrued liabilities      
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Interest rate swaps $ 114,000,000 $ 88,000,000