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SEGMENT INFORMATION - General Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 segment	Feb. 08, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 19, 2017 USD ($)
Reportable segment results
Number of reportable segments	3
Number of coal reportable segments	2
Number of mining complex	7
Equity securities | $			$ 122,094
Kodiak
Reportable segment results
Equity securities | $		$ 0		$ 100,000

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- Definition

Number of coal reportable segments.

+ References

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- Definition

Number of mining complex.

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- Definition

Amount of investment in equity security without readily determinable fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117539-209714

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

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