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Risk Management Activities and Derivative Transactions 1992 Series D Power Bonds (Details) (USD $)
9 Months Ended 12 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Sep. 30, 2003
Jul. 15, 2012
Apr. 15, 2012
Debt Instrument          
Face amount     $ 1,000,000,000    
Proceeds from issuance of swaption     175,000,000    
Interest rate         8.25%
Notional amount       1,000,000,000  
Proceeds from call monetization $ 60,000,000 $ 0