Major Customers (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Concentration Risk [Line Items]
Revenues	$ 56,727	$ 29,930	$ 132,213	$ 100,690
Foresight Energy Lp
Concentration Risk [Line Items]
Revenues	8,552	8,592	25,686	27,052
Contura Energy
Concentration Risk [Line Items]
Revenues	$ 12,854	$ 7,143	$ 29,748	$ 23,164
Sales Revenue \| Customer Concentration Risk \| Foresight Energy Lp
Concentration Risk [Line Items]
Percent	15.00%	29.00%	19.00%	27.00%
Sales Revenue \| Customer Concentration Risk \| Contura Energy
Concentration Risk [Line Items]
Percent	23.00%	24.00%	23.00%	23.00%

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