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Note 4 - Environmental and Reclamation Activities (Details Textual) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 07, 2016 | Aug. 31, 2016 | Dec. 31, 2017 | Sep. 30, 2016 | Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 15, 2015 | Dec. 31, 2014
Proceeds from Insurance Settlement, Investing Activities |  |  |  |  |  | $ 7,745,000 |  |  |  |
Accrued Reclamation and Closure Costs Total |  |  | $ 86,045,000 |  |  | $ 86,045,000 | $ 85,580,000 | $ 95,538,000 |  | $ 57,250,000
Asset Retirement Obligation [Member] |  |  |  |  |  |  |  |  |  |
Fair Value Assumptions, Risk Free Interest Rate |  |  |  |  |  | 5.75% |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |
Inflation Rate, Asset Retirement Obligation |  |  |  |  |  | 2.00% |  |  |  |
Minimum [Member] | Reclamation and Abandonment Costs [Member] |  |  |  |  |  |  |  |  |  |
Fair Value Assumptions, Risk Free Interest Rate |  |  |  |  |  | 5.75% |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |
Inflation Rate, Asset Retirement Obligation |  |  |  |  |  | 4.00% |  |  |  |
Maximum [Member] | Reclamation and Abandonment Costs [Member] |  |  |  |  |  |  |  |  |  |
Fair Value Assumptions, Risk Free Interest Rate |  |  |  |  |  | 14.50% |  |  |  |
Nelson Tunnel/Commodore Site [Member] | United States and Colorado [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingency Accrual, Payments |  | $ 6,000,000 |  |  |  |  |  |  |  |
Gilt Edge Mine Superfund Site [Member] | United States and South Dakota [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingency Accrual, Payments | $ 3,900,000 |  |  |  |  |  |  |  |  |
Gilt Edge Mine Superfund Site [Member] | United States, South Dakota, Insurers and Another Potentially Responsible Party [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingency Accrual, Payments | $ 6,700,000 |  |  |  |  |  |  |  |  |
Gilt Edge Site and Nelson Tunnel/Commodore Site [Member] |  |  |  |  |  |  |  |  |  |
Liability for Remediation |  |  |  |  | $ 9,900,000 |  |  |  |  |
San Sebastian [Member] |  |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Revision of Estimate Before Discounting |  |  |  |  |  | $ 0.50 |  |  |  |
Asset Retirement Obligation, Liabilities Incurred |  |  | 1,200,000 |  |  |  |  |  |  |
Casa Berardi [Member] |  |  |  |  |  |  |  |  |  |
Asset Retirement Obligation, Revision of Estimate Before Discounting |  |  | 1,000,000 |  |  |  |  |  |  |
Asset Retirement Obligation, Liabilities Incurred |  |  | $ 700,000 |  |  |  |  |  |  |
Revett Mining Company, Inc. [Member] |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  | 100.00% |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Reclamation Liability |  |  |  |  | 17,500,000 |  |  |  | $ 17,513,000 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reclamation Insurance Asset |  |  |  |  | $ 16,800,000 |  |  |  | $ 16,800,000 |
Proceeds from Insurance Settlement, Investing Activities |  |  |  | $ 16,000,000 |  |  |  |  |  |
Revett Mining Company, Inc. [Member] | Mining Properties and Mineral Rights [Member] |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  | 100.00% |  |  |  |  |

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