Major Customers (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Concentration Risk [Line Items]
Revenues	$ 37,092	$ 40,214
Foresight Energy Lp
Concentration Risk [Line Items]
Revenues	8,572	8,661
Contura Energy
Concentration Risk [Line Items]
Revenues	$ 8,043	$ 8,571
Sales Revenue \| Customer Concentration Risk \| Foresight Energy Lp
Concentration Risk [Line Items]
Percent	23.00%	22.00%
Sales Revenue \| Customer Concentration Risk \| Contura Energy
Concentration Risk [Line Items]
Percent	22.00%	21.00%

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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+ Details