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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Summary of Derivative Instruments That Receive Hedge Accounting Treatment    
Net unrealized gain (loss) on cash flow hedges $ 33 $ 42
Reclassification to earnings from cash flow hedges (5) [1] 3 [1]
Ineffective portion excluded from testing $ 0 $ 0
[1] There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented.