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, 2019	Mar. 31, 2018
CIBC [Member]
Significant metal customers	16.00%	47.00%
Scotia [Member]
Significant metal customers	29.00%	2.00%
Korea Zinc [Member]
Significant metal customers	20.00%	23.00%
Teck Metals Ltd [Member]
Significant metal customers	15.00%	4.00%
Trafigura AG [Member]
Significant metal customers	12.00%
Ocean Partners [Member]
Significant metal customers		10.00%