Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 296,258	$ 245,515
Less allowances for depreciation, depletion and amortization	123,841	107,454
Property, plant and equipment, net	172,417	138,061
Depreciation, depletion and amortization	15,500	13,600
Coal lands and real estate
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	50,887	54,647
Mineral interests
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	14,181	0
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 231,190	$ 190,868