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Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) oz T | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Capitalized Drilling Costs |  | $ 9,900 | $ 6,800 | $ 6,700
Foreign Currency Transaction Gain (Loss), Realized |  | (10,300) | $ (2,926) | $ 24,551
Restricted Cash and Cash Equivalents |  | 1,000 |  |
Scenario, Forecast [Member] | Accounting Standards Update 2017-07 [Member] |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Except Service cost | $ 2,800 |  |  |
Equity Securities [Member] |  |  |  |
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax |  | $ 3,700 |  |
Silver [Member] |  |  |  |
Metals Contained in Concentrates | oz |  | 1,600,000 |  |
Gold [Member] |  |  |  |
Metals Contained in Concentrates | oz |  | 5,799 |  |
Zinc [Member] |  |  |  |
Metals Contained in Concentrates | T |  | 11,365 |  |
Lead [Member] |  |  |  |
Metals Contained in Concentrates | T |  | 2,705 |  |
Minimum [Member] |  |  |  |
Property, Plant and Equipment, Useful Life |  | 3 years |  |
Maximum [Member] |  |  |  |
Property, Plant and Equipment, Useful Life |  | 22 years |  |

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