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Note 7 - Business Segments and Sales of Products - Sales From Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Sales Revenue, Goods, Net [Member] - Customer Concentration Risk [Member] | 3 Months Ended | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
CIBC [Member] |  |  |  |
Significant metal customers | 26.00% | 23.00% | 34.00% | 24.00%
Scotia [Member] |  |  |  |
Significant metal customers | 19.00% | 33.00% | 13.00% | 26.00%
Korea Zinc [Member] |  |  |  |
Significant metal customers | 15.00% | 25.00% | 22.00% | 20.00%
Teck Metals Ltd [Member] |  |  |  |
Significant metal customers | 11.00% |  | 11.00% | 10.00%
Trafigura AG [Member] |  |  |  |
Significant metal customers | 10.00% |  | 3.00% | 4.00%
Louis Dreyfus [Member] |  |  |  |
Significant metal customers |  | 12.00% |  | 4.00%

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