Mineral Rights, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total depletion and amortization expense on mineral interests	$ 8,800	$ 12,100	$ 17,000
Gain on asset sales and disposals	581	6,498	$ 2,441
Mineral rights, net	460,373	605,096
Coal properties
Mineral rights, net	438,850	560,904
Coal properties | Reported Value Measurement
Mineral rights, net		0
Oil and gas royalty properties
Mineral rights, net	3,761	4,508
Aggregates properties
Mineral rights, net	$ 6,220	$ 28,129