Derivative Financial Instruments - Summary of Mark-to-Market Gain (Losses) on Derivative Instruments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) $ / oz	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / oz	Jun. 30, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Fair value adjustments, net, pretax	$ (2,462,000)	$ 336,000	$ 2,192,000	$ (864,000)
Fair value adjustments, net	$ (242,000)	(763,000)	$ 155,000	449,000
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 silver and gold sales contracts
Derivative Instruments, Gain (Loss) [Line Items]
Provisional gain (loss) on derivatives and commodity contracts	$ (273,000)	(763,000)	$ (20,000)	449,000
Zinc options
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative gain (loss)	219,000	0	363,000	0
Variable interest rate swap receivable	$ 363,000		$ 363,000
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)	$ (188,000)	$ 0	$ (188,000)	$ 0