Inventories, net (Details) - USD ($) $ in Thousands	3 Months Ended
Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Inventory [Line Items]
Total inventories, net		$ 124,245	$ 162,659
Allowance for obsolete materials and supplies inventory	$ 807
Coal
Inventory [Line Items]
Raw coal		18,248	30,274
Saleable coal		79,194	105,092
Material, supplies and other, net
Inventory [Line Items]
Materials, supplies and other, net		$ 26,803	$ 27,293

Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of merchandise or goods held by the company that are readily available for sale.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

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Amount of loss from reductions in inventory due to subsequent measurement adjustments, including, but not limited to, physical deterioration, obsolescence, or changes in price levels.

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