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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, 2013
Sep. 30, 2012
Sep. 30, 2011
Summary of Derivative Instruments That Receive Hedge Accounting Treatment      
Net unrealized gain (loss) on future cash flow hedges $ 78,000,000 $ 99,000,000 $ (50,000,000)
Reclassification to earnings from cash flow hedges (1,000,000) (35,000,000) 7,000,000
Ineffective portion excluded from testing $ 0 [1] $ 0  
[1] There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented.