Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Finished goods	$ 452,958	$ 404,239
Raw materials and work in process	156,723	132,891
Stores, supplies and other	55,841	55,651
Total inventories	$ 665,522	$ 592,781