Derivative Financial Instruments - Summary of Mark-to-Market Gain (Losses) on Derivative Instruments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) $ / oz	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / oz	Sep. 30, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Fair value adjustments, net, pretax	$ 715,000	$ 0	$ 2,907,000	$ (864,000)
Fair value adjustments, net	$ 465,000	147,000	$ 621,000	596,000
Zinc put options purchased
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Weighted Average Strike Price of Put Options | $ / oz	3,000.00		3,000.00
Provisional metal sales contracts
Derivative Instruments, Gain (Loss) [Line Items]
Provisional gain (loss) on derivatives and commodity contracts	$ 34,000	147,000	$ 15,000	596,000
Zinc options
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gain (loss)	$ 225,000	0	$ 588,000
Zinc call options sold
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Weighted Average Strike Price of Call Options	4,050		4,050
Interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gain (loss)	$ 206,000	$ 0	$ 18,000	$ 0