Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Land	$ 118,428	$ 120,842
Land improvements	63,328	59,387
Buildings and improvements	245,482	256,603
Machinery and equipment	2,627,667	2,501,481
Long-term mineral rights and production equipment costs	675,832	654,006
Construction in progress	393,598	318,203
Total	$ 4,124,335	$ 3,910,522