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Inventories, Net (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Sep. 30, 2018
Inventories, Net    
Materials and supplies inventory $ 734 $ 725
Fuel inventory 318 266
Renewable energy certificates/emission allowance inventory, net 16 14
Allowance for inventory obsolescence (52) (44)
Inventories, net $ 1,016 $ 961