Note 11 - Business Segments 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Teck Metals Ltd [Member]
Significant metal customers	11.30%	21.20%	22.50%
Korea Zinc [Member]
Significant metal customers	16.40%	16.90%	16.00%
Scotia [Member]
Significant metal customers	24.90%	24.50%	5.60%
CIBC [Member]
Significant metal customers	22.80%	19.00%	17.60%
Mitsui [Member]
Significant metal customers	7.60%	4.40%	10.20%