Discontinued Operations - Schedule of Major Components of Net (Loss) from Discontinued Operations (Details) - Sands Hill Mining LLC [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues		$ 1,847		$ 3,853
Cost of operations (exclusive of depreciation, depletion and amortization shown separately below)		1,721		3,422
Depreciation, depletion and amortization		139		330
Selling, general and administrative (exclusive of depreciation, depletion and amortization shown separately above)		14		26
Freight and handling costs		228		403
(Gain) on sale/disposal of assets, net				(2)
Total costs and expenses		2,102		4,179
(Loss) from discontinued operations before income taxes for the Sands Hill Mining disposal		(255)		(326)
Income taxes
Net (loss) from discontinued operations		(255)		(326)
Coal Sales [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues		411		937
Limestone Sales [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues		1,007		2,085
Other Revenues [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues		$ 429		$ 831