Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 oz $ / oz	Dec. 31, 2015 oz $ / oz
Silver, 2016 Settlements [Member]
Ounces/pounds under contract | oz	1,124	1,368
Average price per ounce/pound | $ / oz	19.65	14.12
Gold, 2016 Settlements [Member]
Ounces/pounds under contract | oz	2	5
Average price per ounce/pound | $ / oz	1,338	1,076
Zinc, 2016 Settlements [Member]
Ounces/pounds under contract | oz	25,739	23,755
Average price per ounce/pound | $ / oz	1.03	0.71
Lead, 2016 Settlements [Member]
Ounces/pounds under contract | oz	13,117	8,433
Average price per ounce/pound | $ / oz	0.92	0.77
Zinc, 2017 Settlements [Member]
Ounces/pounds under contract | oz	3,858
Average price per ounce/pound | $ / oz	1.08