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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Concentrate Sales Contracts        
Derivative [Line Items]        
Provisional gain (loss) on derivatives and commodity contracts $ 34 $ 147 $ 15 $ 596
Zinc options        
Derivative [Line Items]        
Net derivative gain (loss) 225 0 $ 588  
Interest rate swap        
Derivative [Line Items]        
Term of contract     12 months  
Net derivative gain (loss) $ 206 $ 0 $ 18 $ 0