Income and Mining Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Gross unrecognized benefits	3.8	4.3	5.1
Income tax penalties and interest expense	$ 3,500	$ 4,800	$ 5,500
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	2,500
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	3,500
Zinc options
Income Tax Contingency [Line Items]
Net derivative gain (loss)	$ 753	$ 0	$ 0