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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details) (USD $)
 3 Months Ended 9 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Sep. 30, 2011
Derivative          
Amount of gain (loss) recognized in income on derivatives $ 0   $ 0    
Unrealized gain (loss) on derivatives 0 0 0 0  
Coal Contract Derivatives
          
Derivative          
Fair value (331,000,000)   (331,000,000)   239,000,000
Number of contracts 18   18   38
Notional amount 53,000,000   53,000,000   66,000,000
Maximum number of years left on the derivative terms 6   6    
Swaption
          
Derivative          
Unrealized losses on investments 325,000,000   186,000,000    
Realized gains (losses) 0 0 0 0  
Amount of gain (loss) recognized in income on derivatives 0 0 0 0  
Interest Rate Swap
          
Derivative          
Amount of gain (loss) recognized in income on derivatives 0 0 0 0  
Commodity Contract Derivatives
          
Derivative          
Fair value (331,000,000)   (331,000,000)   239,000,000
Amount of gain (loss) recognized in income on derivatives 0 0 0 0  
Commodity Derivatives Under Financial Trading Program
          
Derivative          
Fair value (391,000,000)   (391,000,000)   (234,000,000)
Amount of gain (loss) recognized in income on derivatives (104,000,000) (29,000,000) (248,000,000) (106,000,000)  
Natural Gas Contract Derivatives
          
Derivative          
Fair value $ 0   $ 0   $ 0
Number of contracts 24   24   13
Notional amount 75,000,000   75,000,000   5,000,000
Maximum number of years left on the derivative terms 3   3