Note 12 - Business Segments, Sales of Products and Significant Customers - Sales from Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Sales Revenue, Goods, Net [Member] - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Teck Metals Ltd [Member]
Significant metal customers	8.40%	11.30%	21.20%
Korea Zinc [Member]
Significant metal customers	17.80%	16.40%	16.90%
Scotia [Member]
Significant metal customers	15.30%	24.90%	24.50%
CIBC [Member]
Significant metal customers	29.70%	22.80%	19.00%