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Derivative Financial Instruments - Summary of Mark-to-Market Gain (Losses) on Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]    
Fair value adjustments, net, pretax $ 9,120 $ 4,654
Fair value adjustments, net 204 398
Provisional metal sales contracts    
Derivative Instruments, Gain (Loss) [Line Items]    
Provisional gain (loss) on derivatives and commodity contracts 250 253
Zinc options    
Derivative Instruments, Gain (Loss) [Line Items]    
Net derivative gain (loss) 0 145
Interest rate swap    
Derivative Instruments, Gain (Loss) [Line Items]    
Net derivative gain (loss) $ (46) $ 0