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Inventories, Net (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Sep. 30, 2019
Inventories, Net    
Materials and supplies inventory $ 773 $ 742
Fuel inventory 328 294
Renewable energy certificates/emission allowance inventory, net 18 16
Allowance for inventory obsolescence (48) (53)
Inventories, net $ 1,071 $ 999