Note 6 - Business Segments and Sales of Products - Sales of Products (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Sales of products	$ 199,703	$ 161,532	$ 502,983	$ 448,321
Less: Smelter and refining charges	(18,587)	(5,351)	(52,177)	(21,609)
Sales by significant product type	209,262	160,484	515,861	448,458
Sales by significant product type	209,262	160,484	515,861	448,458
Silver Contracts [Member]
Sales of products	79,684	40,588	179,013	122,392
Doré and Metals from Doré [Member]
Sales by significant product type	67,659	87,525	211,439	231,391
Sales by significant product type	67,659	87,525	211,439	231,391
Gold [Member]
Sales of products	98,457	103,889	278,363	261,734
Carbon [Member]
Sales by significant product type	23,835	17,745	43,099	27,376
Sales by significant product type	23,835	17,745	43,099	27,376
Lead [Member]
Sales of products	13,370	7,114	32,244	22,809
Lead Concentrate [Member]
Sales by significant product type	85,511	37,685	192,179	122,727
Sales by significant product type	85,511	37,685	192,179	122,727
Zinc [Member]
Sales of products	26,779	15,292	65,540	62,995
Zinc Concentrate [Member]
Sales by significant product type	24,322	12,345	51,727	51,876
Sales by significant product type	24,322	12,345	51,727	51,876
Bulk Concentrate [Member]
Sales by significant product type	7,935	5,184	17,417	15,088
Sales by significant product type	$ 7,935	$ 5,184	$ 17,417	$ 15,088