Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Finished goods	$ 453,662	$ 404,239
Raw materials and work in process	154,679	132,891
Stores, supplies and other	58,226	55,651
Total inventories	$ 666,567	$ 592,781