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Note 6 - Business Segments and Sales of Products - 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] | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
CIBC [Member] |  |
Significant metal customers | 30.00% | 16.00%
Korea Zinc [Member] |  |
Significant metal customers | 19.00% | 20.00%
Asahi [Member] |  |
Significant metal customers | 17.00% | 2.00%
Scotia [Member] |  |
Significant metal customers | 15.00% | 29.00%
IXM [Member] |  |
Significant metal customers | 10.00% | 0.00%
Teck Metals Ltd [Member] |  |
Significant metal customers | 2.00% | 15.00%
Trafigura [Member] |  |
Significant metal customers | 0.00% | 12.00%

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 42 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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