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Inventories (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Inventories      
Finished goods $ 625 $ 872 $ 836
Raw materials 121 128 134
Operating supplies 36 45 45
Inventories $ 782 $ 1,045 $ 1,015