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Note 10 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands | 12 Months Ended
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Dec. 31, 2017 lb oz $ / oz $ / lb | Dec. 31, 2016 lb oz $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 1,447 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 16.64 |
Silver 2017 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  | 1,295
Average price per ounce/pound (in USD per Ounce) | $ / oz |  | 16.29
Gold 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 5 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,279 |
Gold 2017 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  | 4
Average price per ounce/pound (in USD per Ounce) | $ / oz |  | 1,172
Zinc 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 21,550 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.45 |
Zinc 2017 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 19,070
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.18
Lead 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 4,740 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.11 |
Lead 2017 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 7,441
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 0.97
Zinc 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 32,187 | 13,779
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.29 | 1.21
Zinc 2017 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 35,384
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.19
Lead 2018 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 16,645 | 5,732
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.06 | 1.05
Lead 2017 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 17,637
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.03
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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