Other Long-Term Assets (Schedule of Other Long-Term Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Long-term assets:
Advance coal royalties	$ 10,186	$ 9,720
Marketable equity securities available for sale, at fair value (restricted)	1,909	2,148
Other	2,122	2,792
Total other assets	$ 14,217	$ 14,660