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Note 12 - Self-insurance (Details Textual) $ in Thousands | Jun. 30, 2020 USD ($) item | Dec. 31, 2019 USD ($) | Jun. 30, 2019 USD ($)
--- | --- | --- | ---
Property, Plant and Equipment, Gross, Ending Balance | $ 560,428 | $ 550,496 |
Restricted Cash and Cash Equivalents, Total | $ 4,508 | 4,512 | $ 4,576
Future Workers' Compensation Claim Payments [Member] |  |  |
Restricted Cash and Cash Equivalents, Total |  | 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 |

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- Definition Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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