Note 15 - Derivatives (Details Textual) $ in Thousands, T in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) T	Sep. 30, 2020 USD ($)
Unrealized Gain (Loss) on Derivatives and Commodity Contracts, Total	$ (147,306)	$ 0	$ (167,743)	$ 0
Coal Contract [Member]
Derivative, Nonmonetary Notional Amount, Mass (US Ton) \| T			2
Unrealized Gain (Loss) on Derivatives and Commodity Contracts, Total	$ 147,306		$ 167,743

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

Reference 1: 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 http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

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