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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Concentrate Sales Contracts        
Derivative [Line Items]        
Provisional gain (loss) on derivatives and commodity contracts $ 111 $ 631   $ (239)
Zinc options        
Derivative [Line Items]        
Net derivative gain (loss) $ 753 0 $ 0  
Interest rate swap        
Derivative [Line Items]        
Term of contract 12 months      
Net derivative gain (loss) $ (60) $ 0 $ 0