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Risk Management Activities and Derivative Transactions Derivative Instruments That Receive Hedge Accounting Treatment (Details) (USD $)
 3 Months Ended 6 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Mar. 31, 2012
Summary of Derivative Instruments That Receive Hedge Accounting Treatment        
Net unrealized gain (loss) on cash flow hedges $ (49,000,000) $ 21,000,000 $ (16,000,000) $ 64,000,000
Reclassification to earnings from cash flow hedges 63,000,000 [1] (28,000,000) [1] 58,000,000 [1] (26,000,000) [1]
Ineffective portion excluded from testing $ 0 $ 0 $ 0 $ 0
[1] There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented.