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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, 2015
Sep. 30, 2014
Sep. 30, 2013
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Net unrealized gain (loss) on future cash flow hedges $ (72,000,000) $ 4,000,000 $ 78,000,000
Reclassification to earnings from cash flow hedges 65,000,000 (2,000,000) $ (1,000,000)
Ineffective portion excluded from testing 0 $ 0  
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred $ (14,000,000)