Property, Plant and Equipment - Schedule of Depreciation, Depletion, and Amortization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property, Plant and Equipment [Line Items]
Total depreciation, depletion and amortization	$ 5,677	$ 5,470	$ 11,104	$ 10,977
Mining and Other Equipment and Related Facilities [Member]
Property, Plant and Equipment [Line Items]
Total depreciation, depletion and amortization	4,244	4,220	8,331	8,480
Coal Properties [Member]
Property, Plant and Equipment [Line Items]
Total depreciation, depletion and amortization	481	415	953	770
Mine Development Costs [Member]
Property, Plant and Equipment [Line Items]
Total depreciation, depletion and amortization	821	788	1,570	1,545
Asset Retirement Costs [Member]
Property, Plant and Equipment [Line Items]
Total depreciation, depletion and amortization	$ 131	$ 47	$ 250	$ 182