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Note 12- Business Segments, Sales of Products and Significant Customers - Sales from Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Customer Concentration Risk [Member] - Revenue from Contract with Customer Benchmark [Member] | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
CIBC [Member] |  |  |
Significant metal customers | 32.70% | 23.10% | 29.70%
Teck Metals Ltd [Member] |  |  |
Significant metal customers | 16.10% | 8.20% | 8.40%
Ocean Partners [Member] |  |  |
Significant metal customers | 13.90% | 5.70% | 2.50%
Korea Zinc [Member] |  |  |
Significant metal customers | 13.30% | 17.40% | 17.80%
Scotia [Member] |  |  |
Significant metal customers | 2.90% | 24.00% | 15.30%

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