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Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Tax Contingency [Line Items] |  |  |
Gross unrecognized benefits | 3.8 | 4.3 | 5.1
Income tax penalties and interest expense | $ 3,500 | $ 4,800 | $ 5,500
Minimum |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized income tax liability | 2,500 |  |
Maximum |  |  |
Income Tax Contingency [Line Items] |  |  |
Unrecognized income tax liability | 3,500 |  |
Zinc options |  |  |
Income Tax Contingency [Line Items] |  |  |
Net derivative gain (loss) | $ 753 | $ 0 | $ 0

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