LONG-LIVED ASSET IMPAIRMENTS (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)
Asset impairment charges
Asset impairments	$ 24,977
Illinois Basin
Asset impairment charges
Asset impairments	23,500
Coal Royalties
Asset impairment charges
Asset impairments	$ 1,500

Amount recognized as an operating expense or loss during the period to reduce the carrying amount of a project that has been impaired but not abandoned.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI https://asc.fasb.org/extlink&oid=126905981&loc=d3e2420-110228

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration