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Derivative Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Derivative [Line Items]        
Provisional gain (loss) on derivatives and commodity contracts $ 79 $ (962) $ (618) $ 250
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months     4,600  
Designated as Hedging Instrument | Gold zero cost collars        
Derivative [Line Items]        
After tax gains in AOCI     $ 4,900