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Fair Value Measurement - Statement Of Income Classification (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Fair Value Statement Of Income Classification [Line Items] |  |  |  |  |
Derivative gain (loss) |  |  | $ (2,300) |  | $ 400
Sales |  |  |  |  |
Fair Value Statement Of Income Classification [Line Items] |  |  |  |  |
Derivative gain (loss) | $ (2,303) | $ (24) | (1,732) | $ 511 |
Other Operating Income (Expense) |  |  |  |  |
Fair Value Statement Of Income Classification [Line Items] |  |  |  |  |
Gold and silver rounds/bullion gain (loss) | $ (160) | $ (148) | $ (144) | $ 154 |

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- Definition Fair Value Statement Of Income Classification [Line Items] + References No definition available. + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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