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Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Sep. 30, 2012
Natural Gas Futures
          
Derivative          
Notional amount 0   0   0
Fair value $ 0   $ 0   $ 0
Fuel Oil and Crude Oil Futures
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Coal Futures
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Natural Gas Swap
          
Derivative          
Notional amount 165,662,500   165,662,500   294,462,500
Fair value (179)   (179)   (232)
Fuel Oil and Crude Oil Swap
          
Derivative          
Notional amount 1,313,000   1,313,000   1,390,000
Fair value (2)   (2)   4
Coal Swap
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Natural Gas Option
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Fuel Oil and Crude Oil Option
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Coal Option
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Natural Gas Total Contracts
          
Derivative          
Notional amount 165,662,500   165,662,500   294,462,500
Fair value (179)   (179)   (232)
Decrease (increase) in fuel expense (13) (53) (60) (69)  
Decrease (increase) in purchased power expense (8) (53) (40) (189)  
Fuel Oil and Crude Oil Total Contracts
          
Derivative          
Notional amount 1,313,000   1,313,000   1,390,000
Fair value (2)   (2)   4
Decrease (increase) in fuel expense 0 1 2 9  
Coal Contract Total Contracts
          
Derivative          
Notional amount 0   0   0
Fair value 0   0   0
Decrease (increase) in fuel expense 0 0 (1) 1  
Closed derivative contracts
          
Derivative          
Fair value $ (9)   $ (9)   $ (21)