Note 4 - Other Long-term Assets - Other Long-term Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Advanced coal royalties	$ 6,481	$ 6,449
Other	1,780	1,809
Total other assets	$ 8,261	$ 8,258

Carrying amount as of the balance sheet date of capitalized amounts paid for mineral royalties which will be charged against earnings after one year or beyond the normal operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Name:	us-gaap_PrepaidMineralRoyaltiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant