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Inventories (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Inventories      
Finished goods $ 868 $ 849 $ 901
Raw materials 126 125 120
Operating supplies 44 44 44
Inventories $ 1,038 $ 1,018 $ 1,065