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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Concentrate Sales Contracts        
Derivative [Line Items]        
Provisional gain (loss) on derivatives and commodity contracts $ (273) $ (763) $ (20) $ 449
Zinc options        
Derivative [Line Items]        
Net derivative gain (loss) 219 0 $ 363 0
Interest rate swap        
Derivative [Line Items]        
Term of contract     12 months  
Net derivative gain (loss) $ (188) $ 0 $ (188) $ 0