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Inventories, Net (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Sep. 30, 2016
Inventories, Net    
Materials and supplies inventory $ 728 $ 673
Fuel inventory 410 345
Emission allowance inventory, net 15 14
Allowance for inventory obsolescence (33) (39)
Inventories, net $ 1,120 $ 993