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Derivative Assets and Derivative Liabilities (Schedule of Derivative Gain (Loss) Settled and Open Positions) (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) $ / T | Dec. 31, 2016 USD ($) $ / T
Zinc Commodity [member] | Option and Forward Contracts [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Tonnes settled | 7,803 |
Average settled price per tonne | 2,894 |
Settlement gains (losses) | $ | $ (1,521) |
Open positions - tonnes | 6,500 | 7,803
Unrealized gains (losses) | $ | $ (2,957) | $ 973
Zinc Commodity [member] | Option and Forward Contracts [member] | Bottom of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne | 2,500 | 2,650
Zinc Commodity [member] | Option and Forward Contracts [member] | Top of range [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Price per tonne | 3,190 | 2,750
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 |
Lead Commodity [member] | Option and Forward Contracts [member] |  |
Disclosure of financial assets and liabilities [line items] |  |
Tonnes settled | 4,465 |
Average settled price per tonne | 2,361 |
Settlement gains (losses) | $ | $ 19 |
Open positions - tonnes | 6,000 |
Unrealized gains (losses) | $ | $ (344) |

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