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Inventories, Net (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Sep. 30, 2020
Inventories, Net    
Materials and supplies inventory $ 783 $ 770
Fuel inventory 246 253
Renewable energy certificates/emission allowance inventory, net 12 15
Allowance for inventory obsolescence (35) (35)
Inventories, net $ 1,006 $ 1,003