Note 13 - Self-insurance (Details Textual) $ in Thousands	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)
Property, Plant and Equipment, Gross, Ending Balance	$ 561,603	$ 550,496
Restricted Cash and Cash Equivalents, Total	4,030	4,512
Future Workers' Compensation Claim Payments [Member]
Restricted Cash and Cash Equivalents, Total	$ 4,000	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	$ 269,000	$ 273,000