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Derivative Assets and Derivative Liabilities (Schedule of Derivative Gain (Loss) Settled and Open Positions) (Details) - Option and Forward Contracts [member] $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) $ / T | Dec. 31, 2017 USD ($) $ / T
Zinc Commodity [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Tonnes settled | 8,000 | 7,803
Average settled price per tonne | 2,925 | 2,894
Settlement gains (losses) | $ | $ (49) | $ (1,521)
Open positions - tonnes | 4,500 | 6,500
Unrealized gains (losses) | $ | $ 4,630 | $ (2,957)
Zinc Commodity [member] | Bottom of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne | 3,050 | 2,500
Zinc Commodity [member] | Top of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne | 3,300 | 3,190
Lead Commodity [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Tonnes settled | 6,000 | 4,465
Average settled price per tonne | 2,302 | 2,361
Settlement gains (losses) | $ | $ 443 | $ 19
Open positions - tonnes |  | 6,000
Unrealized gains (losses) | $ | $ 344 | $ (344)
Lead Commodity [member] | Bottom of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne |  | 2,100
Lead Commodity [member] | Top of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne |  | 2,689

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