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Note 11 - Self-insurance (Details Textual) $ in Thousands | Sep. 30, 2021 USD ($) item | Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($)
--- | --- | --- | ---
Restricted Cash and Cash Equivalents, Total | $ 3,249 | $ 4,030 | $ 4,243
Future Workers' Compensation Claim Payments [Member] |  |  |
Restricted Cash and Cash Equivalents, Total | $ 3,200 | 4,000 |
Mining Properties and Mineral Rights [Member] |  |  |
Number of Mining Units | 7 |  |
Area of Real Estate Property | item | 10 |  |
Operating Lease, Liability, Total | $ 278,000 | $ 269,000 |

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