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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Concentrate Sales Contracts    
Derivative [Line Items]    
Provisional gain (loss) on derivatives and commodity contracts $ 500 $ 250
Interest rate swap    
Derivative [Line Items]    
Net derivative gain (loss) 0 $ (46)
Designated as Hedging Instrument | Gold zero cost collars    
Derivative [Line Items]    
After tax gains in AOCI $ 100