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Inventories, Net (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Inventories, Net    
Materials and supplies inventory $ 758 $ 734
Fuel inventory 322 355
Renewable energy certificates/emission allowance inventory, net 12 15
Allowance for inventory obsolescence (45) (39)
Inventories, net $ 1,047 $ 1,065