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Inventories, Net (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Sep. 30, 2017
Inventories, Net    
Materials and supplies inventory $ 775 $ 734
Fuel inventory 304 355
Renewable energy certificates/emission allowance inventory, net 13 15
Allowance for inventory obsolescence (42) (39)
Inventories, net $ 1,050 $ 1,065