Note 10 - Derivative Instruments (Details Textual) $ in Thousands, MXN in Millions, CAD in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2017 MXN
Maximum Allocation of Forecasted CAD-demonimated Operating Costs	75.00%			75.00%	75.00%
Forecasted CAD-denominated Operating Costs to be Hedged, Term	5 years
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	$ 5,000
Gain (Loss) on Sale of Derivatives	(21,250)	$ 4,423	$ 8,252
Gain (Loss) on Components Excluded from Assessment of Foreign Currency Cash Flow Hedge Effectiveness	5
Derivative, Loss on Derivative	6,000	$ 14,200
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	800
Derivative, Fair Value, Obligations Under the Agreements	800
Cost of Sales [Member]
Gain (Loss) on Sale of Derivatives	1,100
Other Comprehensive Income (Loss) [Member]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	2,100
Foreign Exchange Contract [Member] | Other Current Assets [Member]
Derivative Asset, Current	2,300
Foreign Exchange Contract [Member] | Other Current Liabilities [Member]
Derivative Liability, Current	400
Foreign Exchange Contract [Member] | Other Noncurrent Assets [Member]
Derivative Asset, Noncurrent	2,700
Foreign Exchange Contract [Member] | Other Noncurrent Liabilities [Member]
Derivative Liability, Noncurrent	$ 300
Foreign Exchange Contract [Member] | Casa Berardi [Member] | Maximum [Member] | United States of America, Dollars
Derivative, Forward Exchange Rate	1.338			1.338	1.338
Foreign Exchange Contract [Member] | Casa Berardi [Member] | Designated as Hedging Instrument [Member]
Derivative, Number of Instruments Held	96			96	96
Derivative, Notional Amount	$ 164,500			CAD 213.1
Foreign Exchange Contract [Member] | Casa Berardi [Member] | Designated as Hedging Instrument [Member] | Minimum [Member]
Derivative, Forward Exchange Rate	1.2787			1.2787	1.2787
Foreign Exchange Contract [Member] | San Sebastian [Member] | Minimum [Member] | United States of America, Dollars
Derivative, Forward Exchange Rate	18.8317			18.8317	18.8317
Foreign Exchange Contract [Member] | San Sebastian [Member] | Maximum [Member] | United States of America, Dollars
Derivative, Forward Exchange Rate	20.855			20.855	20.855
Foreign Exchange Contract [Member] | San Sebastian [Member] | Designated as Hedging Instrument [Member]
Derivative, Number of Instruments Held	25			25	25
Derivative, Notional Amount	$ 14,100				MXN 278.2
Commodity Contract [Member] | Other Current Liabilities [Member]
Derivative Liability, Current	10,300
Derivative Liability, Fair Value, Gross Asset	200
Commodity Contract [Member] | Other Noncurrent Liabilities [Member]
Derivative Liability, Noncurrent	5,200
Unsettled Concentrate Sales Contracts [Member]
Derivative, Loss on Derivative	6,000
Forecasted Future Concentrate Contracts [Member]
Derivative, Loss on Derivative	$ 21,300