Note 6 - Business Segments and Sales of Products - Sales from Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Customer Concentration Risk [Member] - Revenue from Contract with Customer Benchmark [Member]	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
CIBC [Member]
Significant metal customers	30.00%	16.00%
Korea Zinc [Member]
Significant metal customers	19.00%	20.00%
Asahi [Member]
Significant metal customers	17.00%	2.00%
Scotia [Member]
Significant metal customers	15.00%	29.00%
IXM [Member]
Significant metal customers	10.00%	0.00%
Teck Metals Ltd [Member]
Significant metal customers	2.00%	15.00%
Trafigura [Member]
Significant metal customers	0.00%	12.00%