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Inventories (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Inventories      
Finished goods $ 875 $ 827 $ 892
Raw materials 129 118 127
Operating supplies 42 38 38
Inventories $ 1,046 $ 983 $ 1,057