Other Long-Term Assets and Other Income (Schedule of Other Long-Term Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term assets:
Advance coal royalties	$ 9,720	$ 9,296
Marketable equity securities available for sale, at fair value (restricted)	2,148	2,036
Other	2,792	2,782
Total other assets	$ 14,660	$ 14,114