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Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands | 12 Months Ended
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Dec. 31, 2018 lb oz $ / oz $ / lb | Dec. 31, 2017 lb oz $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 842 | 1,447
Average price per ounce/pound (in USD per Ounce) | $ / oz | 14.69 | 16.64
Gold 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 4 | 5
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,260 | 1,279
Zinc 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 18,450 | 21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.15 | 1.45
Lead 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 2,700 | 4,740
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.89 | 1.11
Zinc 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 32,187
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.29
Lead 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 16,645
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.06
Zinc 2019 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 23,589
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.33
Lead 2019 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 18,078
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.09
Zinc 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 3,307
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.27
Lead 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 2,866
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.08

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- Definition Specified interest rate, security price, commodity price, foreign exchange rate, index of prices or rates for a derivative contract measured in mass but not the derivative contract itself. + References No definition available. + Details
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