Mineral Rights, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Proceeds from sale of assets									$ (2,449)	$ (1,151)	$ (62,117)
Gain (loss) on asset sales	$ 1,622	$ 0	$ 168	$ 651	$ 178	$ 154	$ 3,184	$ 29	2,441	3,545	29,068
Total depletion and amortization expense on mineral interests									17,000	20,100	27,800
Appalachian Basin
Proceeds from sale of assets											36,400
Gain (loss) on asset sales											18,600
Texas, Georgia, Tennessee
Gain (loss) on asset sales											1,500
Proceeds from Sale of Hard Mineral Reserves											10,000
Other Mineral Rights
Proceeds from sale of assets									2,400	(1,000)	(17,300)
Gain (loss) on asset sales									$ 2,400	$ 3,500	$ 8,600