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Note 12 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 oz lb $ / oz $ / lb | Dec. 31, 2017 oz lb $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 1,221 | 1,447
Average price per ounce/pound (in USD per Ounce) | $ / oz | 14.56 | 16.64
Gold 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 6 | 5
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,211 | 1,279
Zinc 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 10,362 | 21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.17 | 1.45
Lead 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 4,685 | 4,740
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.94 | 1.11
Silver 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  |
Gold 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  |
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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