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PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	$ 3,554,090	$ 3,684,008
Less accumulated depreciation, depletion and amortization	(1,753,845)	(1,675,022)
Total property, plant and equipment, net	1,800,245	2,008,986
Mining equipment and processing facilities
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	1,896,324	1,937,642
Land and coal mineral rights
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	454,310	453,237
Coal reserves not subject to depletion	37,500	40,100
Oil and gas mineral interests
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	616,904	618,282
Unproved properties	340,500	376,200
Buildings, office equipment and improvements
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	279,938	304,111
Construction and mine development in progress
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	25,799	86,876
Mine development costs
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	280,815	283,860
Mine development costs	$ 13,100	$ 13,200

X

- Definition

Represents the carrying value as of the balance sheet date of coal reserves attributable to properties where the entity is not currently engaged in operations or leasing to third parties and therefore the coal reserves are not being depleted.

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Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table.

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Amount of accumulated depreciation and amortization from plant, property, and equipment and right-of-use asset from finance lease.

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Amount, after accumulated depreciation and amortization, of property, plant, and equipment and finance lease right-of-use asset.

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Amount, before accumulated depreciation and amortization, of property, plant, and equipment and finance lease right-of-use asset.

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