Major Customers (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Revenues	$ 139,739	$ 257,437	$ 251,071
Foresight Energy Lp
Concentration Risk [Line Items]
Revenues	35,704	58,923	54,595
Contura Energy
Concentration Risk [Line Items]
Revenues	$ 33,227	$ 40,743	$ 24,580
Sales Revenue | Customer Concentration Risk | Foresight Energy Lp
Concentration Risk [Line Items]
Concentration risk, percentage	26.00%	23.00%	22.00%
Sales Revenue | Customer Concentration Risk | Contura Energy
Concentration Risk [Line Items]
Concentration risk, percentage	24.00%	16.00%	10.00%