Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 452,158	$ 452,158	$ 382,224
Less allowances for depreciation, depletion, amortization and impairment	228,256	228,256	164,272
Property, plant and equipment, net	223,902	223,902	217,952
Depreciation, depletion and amortization		26,400	23,100
Long-lived asset impairment charge	65,900	65,887	3,939
Coal lands and real estate
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	58,353	58,353	60,277
Mineral interests
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	68,150	68,150	31,436
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, at cost	$ 325,655	$ 325,655	$ 290,511