Note 10 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	12 Months Ended
	Dec. 31, 2017 lb oz $ / oz $ / lb	Dec. 31, 2016 lb oz $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,447
Average price per ounce/pound (in USD per Ounce) | $ / oz	16.64
Silver 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz		1,295
Average price per ounce/pound (in USD per Ounce) | $ / oz		16.29
Gold 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	5
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,279
Gold 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz		4
Average price per ounce/pound (in USD per Ounce) | $ / oz		1,172
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.45
Zinc 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb		19,070
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.18
Lead 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	4,740
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.11
Lead 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb		7,441
Average price per ounce/pound (in USD per Ounce) | $ / lb		0.97
Zinc 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	32,187	13,779
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.29	1.21
Zinc 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		35,384
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.19
Lead 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	16,645	5,732
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.06	1.05
Lead 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb		17,637
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.03
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