Note 8 - Revenue - Domestic Revenues an Export Revenues (Details) $ in Thousands, T in Millions	3 Months Ended
Mar. 31, 2021 USD ($) T $ / T	Mar. 31, 2020 USD ($)
REVENUES
Revenues	$ 43,455	$ 41,935
Term of domestic sales contracts	1 year
Domestic Coal Revenues
REVENUES
Revenues	$ 20,107	30,532
Export Revenues
REVENUES
Revenues	$ 23,348	$ 11,403
Fixed Priced Contracts
REVENUES
Outstanding performance obligation, mass \| T	1.2
Average per ton \| $ / T	87
Contracts with Indexed Based Pricing Mechanisms
REVENUES
Outstanding performance obligation, mass \| T	0.3

Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

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