Mineral Rights, Net - Mineral Rights (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	$ 815,895,000		$ 820,170,000
Accumulated Depletion	(364,285,000)		(359,797,000)
Net Book Value	451,610,000		460,373,000
Asset impairments	4,043,000	$ 0
Coal Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	781,392,000		785,623,000
Accumulated Depletion	(351,037,000)		(346,773,000)
Net Book Value	430,355,000		438,850,000
Asset impairments	4,000,000.0
Aggregates Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	8,995,000		9,039,000
Accumulated Depletion	(2,912,000)		(2,819,000)
Net Book Value	6,083,000		6,220,000
Oil And Gas Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	12,354,000		12,354,000
Accumulated Depletion	(8,724,000)		(8,593,000)
Net Book Value	3,630,000		3,761,000
Other Mineral Rights
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Carrying Value	13,154,000		13,154,000
Accumulated Depletion	(1,612,000)		(1,612,000)
Net Book Value	$ 11,542,000		$ 11,542,000