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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Concentrate Sales Contracts    
Derivative [Line Items]    
Provisional gain (loss) on derivatives and commodity contracts $ 253 $ 1,212
Zinc options    
Derivative [Line Items]    
Net derivative gain (loss) $ 100 $ 0