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Inventories, Net (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Sep. 30, 2011
Inventories, Net    
Materials and supplies inventory $ 606 $ 555
Fuel inventory 563 489
Emission allowance inventory 12 11
Allowance for inventory obsolescence (36) (27)
Inventories, net $ 1,145 $ 1,028