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Inventories (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Inventories      
Finished goods $ 821 $ 873 $ 879
Raw materials 123 122 130
Operating supplies 41 41 40
Inventories $ 985 $ 1,036 $ 1,049