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Note 10 - Derivative Instruments (Details Textual) $ in Thousands, $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2019 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, Ending Balance			$ 300
Foreign Currency Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net			1,300
Gain (Loss) on Components Excluded from Assessment of Foreign Currency Cash Flow Hedge Effectiveness			0
Derivative, Gain (Loss) on Derivative, Net, Total			(5,613)	$ 15,366	$ (20,741)
Derivative, Gain on Derivative				$ 8,100
Derivative Liability, Fair Value, Amount Not Offset Against Collateral, Total			9,300
Derivative, Fair Value, Obligations Under the Agreements			9,300
Other Comprehensive Income (Loss) [Member]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months			100
Foreign Exchange Contract [Member] \| Other Current Assets [Member]
Derivative Asset, Current			600
Foreign Exchange Contract [Member] \| Other Noncurrent Assets [Member]
Derivative Assets, Noncurrent, Total			600
Foreign Exchange Contract [Member] \| Other Current Liabilities [Member]
Derivative Liability, Current			400
Foreign Exchange Contract [Member] \| Other Noncurrent Liabilities [Member]
Derivative Liability, Noncurrent			$ 1,000
Foreign Exchange Contract [Member] \| Casa Berardi [Member] \| Designated as Hedging Instrument [Member]
Derivative, Number of Instruments Held, Total			128			128	128
Derivative, Notional Amount			$ 214,700			$ 279
Foreign Exchange Contract [Member] \| Casa Berardi [Member] \| Designated as Hedging Instrument [Member] \| Minimum [Member]
Derivative, Forward Exchange Rate			1.2702			1.2702	1.2702
Foreign Exchange Contract [Member] \| Casa Berardi [Member] \| Designated as Hedging Instrument [Member] \| Maximum [Member]
Derivative, Forward Exchange Rate			1.3332			1.3332	1.3332
Foreign Exchange Contract [Member] \| San Sebastian [Member] \| Designated as Hedging Instrument [Member]
Derivative, Number of Instruments Held, Total			5			5	5
Derivative, Notional Amount			$ 300				$ 7.1
Foreign Exchange Contract [Member] \| San Sebastian [Member] \| Designated as Hedging Instrument [Member] \| Minimum [Member]
Derivative, Forward Exchange Rate			20.5610			20.5610	20.5610
Foreign Exchange Contract [Member] \| San Sebastian [Member] \| Designated as Hedging Instrument [Member] \| Maximum [Member]
Derivative, Forward Exchange Rate			20.8450			20.8450	20.8450
Commodity Contract [Member]
Derivative Liability, Current			$ 5,800
Derivative Liability, Total			2,100
Forecasted Future Concentrate Contracts [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			(1,300)
Derivative, Gain on Derivative	$ 6,700	$ 32,800
Forecasted Future Concentrate Contracts [Member] \| Forward Contracts [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			(1,000)
Forecasted Future Concentrate Contracts [Member] \| Put Option [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			(300)
Unsettled Concentrate Sales Contracts [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			(4,000)
Unsettled Concentrate Sales Contracts [Member] \| Forward Contracts [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			4,500
Unsettled Concentrate Sales Contracts [Member] \| Deferred Premium on Put Option Contract [Member]
Derivative, Gain (Loss) on Derivative, Net, Total			$ (8,500)