Other Assets (Schedule of Other Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Long-term assets:
Advance coal royalties	$ 10,284	$ 10,186
Marketable equity securities available for sale at fair value (restricted)*	2,181	1,909
Other	1,952	2,122
Total other assets	$ 14,417	$ 14,217