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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, 2018
Sep. 30, 2017
Sep. 30, 2016
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Net unrealized gain (loss) on future cash flow hedges $ 10,000,000 $ 59,000,000 $ (139,000,000)
Reclassification to earnings from cash flow hedges 26,000,000 $ (26,000,000) $ 129,000,000
Ineffective portion excluded from testing 0    
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred $ 28,000,000