Property, Plant and Equipment - Schedule of Property, Plant and Equipment by Major Classification (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total	$ 448,799	$ 440,843
Less accumulated depreciation, depletion and amortization	(270,137)	(263,520)
Net	178,662	177,323
Mining and Other Equipment and Related Facilities [Member]
Property, Plant and Equipment [Line Items]
Total	$ 305,554	298,293
Mining and Other Equipment and Related Facilities [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	2 years
Mining and Other Equipment and Related Facilities [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	20 years
Mine Development Costs [Member]
Property, Plant and Equipment [Line Items]
Total	$ 60,040	58,566
Mine Development Costs [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	1 year
Mine Development Costs [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	15 years
Coal Properties [Member]
Property, Plant and Equipment [Line Items]
Total	$ 64,070	64,070
Coal Properties [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	1 year
Coal Properties [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful Lives	15 years
Land [Member]
Property, Plant and Equipment [Line Items]
Total	$ 14,050	14,687
Construction Work in Process [Member]
Property, Plant and Equipment [Line Items]
Total	$ 5,085	$ 5,227