Inventories (Inventories) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Finished products	$ 488,061	$ 500,861
Feedstock, additives and chemicals	213,144	216,877
Materials and supplies	133,630	83,362
Inventories	$ 834,835	$ 801,100