Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 335,633	$ 296,258
Less allowances for depreciation, depletion and amortization	142,466	123,841
Property, plant and equipment, net	193,167	172,417
Depreciation, depletion and amortization	19,500	15,500
Coal lands and real estate
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	52,011	50,887
Mineral interests
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	19,512	14,181
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 264,110	$ 231,190