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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

SEGMENT INFORMATION - General Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 segment	Feb. 08, 2019 USD ($)	Jul. 19, 2017 USD ($)
Reportable segment results
Number of reportable segments	3
Number of coal reportable segments	2
Number of mining complex	7
Kodiak
Reportable segment results
Equity securities \| $		$ 0.0	$ 100.0