Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Abstract]
Stock-based compensation	$ 512	$ 458
Investment	1,031	827
Net operating loss	14,908	7,583
Alternative minimum tax credit	4,221	4,388
Other	564	18
Total long-term deferred tax assets	21,236	13,274
Coal properties	(50,439)	(46,596)
Oil and gas properties	(15,971)	(15,711)
Total long-term deferred tax liabilities	(66,410)	(62,307)
Net deferred tax liability	$ (45,174)	$ (49,033)