Note 6 - Business Segments and Sales of Products - Sales From Continuing Operations to Significant Metals Customers As a Percentage of Total Sales (Details) - Sales Revenue, Goods, Net [Member] - Customer Concentration Risk [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
CIBC [Member]
Significant metal customers	28.00%	23.00%	38.00%	24.00%
Scotia [Member]
Significant metal customers	18.00%	23.00%	10.00%	22.00%
Korea Zinc [Member]
Significant metal customers	28.00%	14.00%	26.00%	17.00%
Teck Metals Ltd [Member]
Significant metal customers	17.00%	15.00%	11.00%	15.00%
Trafigura AG [Member]
Significant metal customers		12.00%		6.00%