Note 10 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	12 Months Ended
	Dec. 31, 2019 oz lb $ / oz $ / lb $ / item	Dec. 31, 2018 oz lb $ / item
Silver 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	2,556
Average price per ounce/pound (in USD per Ounce) | $ / oz	17.20
Gold 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	10
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,481
Zinc 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.04
Lead 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	5,159
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.88
Silver 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz		842
Average price per ounce/pound (in USD per Ounce) | $ / item		14.69
Gold 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz		4
Average price per ounce/pound (in USD per Ounce) | $ / item		1,260
Zinc 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb		18,450
Average price per ounce/pound (in USD per Ounce) | $ / item		1.15
Lead 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb		2,700
Average price per ounce/pound (in USD per Ounce) | $ / item		0.89
Silver 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	5,700
Average price per ounce/pound (in USD per Ounce) | $ / item	15.73
Gold 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	130
Average price per ounce/pound (in USD per Ounce) | $ / item	1,435
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	441
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.13
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	11,740
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.98