Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Abstract]
Stock-based compensation	$ 251	$ 512
Investment	781	1,031
Net operating loss	13,626	14,908
Alternative minimum tax credit	2,705	4,221
Other	943	564
Total long-term deferred tax assets	18,306	21,236
Coal properties	(47,034)	(50,439)
Oil and gas properties	0	(15,971)
Total long-term deferred tax liabilities	(47,034)	(66,410)
Net deferred tax liability	$ (28,728)	$ (45,174)