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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Collateral (Details) (USD $)
12 Months Ended
Sep. 30, 2013
Credit of Customers
Customers
Sep. 30, 2012
Credit of Customers
Customers
Sep. 30, 2013
Collateral
Derivative      
Number of customers that represent the percent of outstanding accounts receivable 3 3  
Aggregate fair value of derivative instruments with credit-risk related contingent features that were in a liability position     $ 1,200,000,000
Collateral obligations     800,000,000
Collateral already posted     800,000,000
Likely cash collateral obligation increase if credit is downgraded     $ 22,000,000