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Inventories, Net (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Sep. 30, 2014
Inventories, Net    
Materials and supplies inventory $ 635us-gaap_InventoryRawMaterialsAndSupplies $ 616us-gaap_InventoryRawMaterialsAndSupplies
Fuel inventory 520us-gaap_EnergyRelatedInventory 473us-gaap_EnergyRelatedInventory
Emission allowance inventory 16us-gaap_OtherInventory 13us-gaap_OtherInventory
Allowance for inventory obsolescence (36)us-gaap_InventoryValuationReserves (46)us-gaap_InventoryValuationReserves
Inventories, net $ 1,135us-gaap_InventoryNet $ 1,056us-gaap_InventoryNet