Property, Plant and Equipment, Net (Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 500,787	$ 457,847
Less allowances for depreciation, depletion and amortization	213,241	198,390
Property, plant and equipment, net	287,546	259,457
Coal lands and real estate
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	72,490	70,766
Mineral interests
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	73,850	69,148
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 354,447	$ 317,933