Note 12 - Self-insurance (Details Textual) $ in Thousands	Sep. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Property, Plant and Equipment, Gross, Ending Balance	$ 562,621	$ 550,496
Restricted Cash and Cash Equivalents, Total	4,243	4,512	$ 4,726
Future Workers' Compensation Claim Payments [Member]
Restricted Cash and Cash Equivalents, Total	$ 4,200	4,500
Mining Properties and Mineral Rights [Member]
Number of Mining Units	7
Area of Real Estate Property \| item	10
Property, Plant and Equipment, Gross, Ending Balance	$ 273,000	$ 273,000

Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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