Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Inventories
Finished goods	$ 954	$ 858	$ 858
Raw materials	116	113	109
Operating supplies	37	36	40
Inventories	$ 1,107	$ 1,007	$ 1,007