Note 11 - Business Segments, Sales of Products and Significant Customers - Sales of Products (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Sales of products	$ 673,266	$ 567,137	$ 577,775
Less: Smelter and refining charges	(33,004)	(24,177)	(36,477)
Sales by significant product type	674,549	559,053	583,758
Silver Contracts [Member]
Sales of products	192,235	144,609	194,813
Doré and Metals from Doré [Member]
Sales by significant product type	340,912	306,328	292,930
Gold [Member]
Sales of products	388,602	313,076	277,421
Carbon [Member]
Sales by significant product type	37,645	5,239	2,246
Lead [Member]
Sales of products	35,777	33,829	37,995
Lead Concentrate [Member]
Sales by significant product type	200,456	148,880	179,143
Zinc [Member]
Sales of products	89,656	99,800	104,023
Zinc Concentrate [Member]
Sales by significant product type	74,160	80,938	88,688
Bulk Concentrate [Member]
Sales by significant product type	$ 21,376	$ 17,668	$ 20,751