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Inventories, Net (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Sep. 30, 2012
Inventories, Net    
Materials and supplies inventory $ 608 $ 605
Fuel inventory 504 508
Emission allowance inventory 13 12
Allowance for inventory obsolescence (31) (28)
Inventories, net $ 1,094 $ 1,097