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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
CIBC [Member]
Significant metal customers	24.00%	28.00%	19.00%	38.00%
Scotia [Member]
Significant metal customers	28.00%	18.00%	28.00%	10.00%
Korea Zinc [Member]
Significant metal customers	20.00%	28.00%	20.00%	26.00%
Teck Metals Ltd [Member]
Significant metal customers	3.00%	17.00%	9.00%	11.00%
Ocean Partners [Member]
Significant metal customers	12.00%	0.00%	6.00%	0.00%