Note 12 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 oz lb $ / oz $ / lb	Dec. 31, 2017 oz lb $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,221	1,447
Average price per ounce/pound (in USD per Ounce) | $ / oz	14.56	16.64
Gold 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	6	5
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,211	1,279
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	10,362	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.17	1.45
Lead 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	4,685	4,740
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.94	1.11
Silver 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz
Gold 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz
Zinc 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		32,187
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.29
Lead 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		16,645
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.06
Zinc 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		23,589
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.33
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		18,078
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.09
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		3,307
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.27
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		2,866
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.08