Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details)	12 Months Ended
	Dec. 31, 2020 oz lb $ / oz $ / lb	Dec. 31, 2019 oz lb $ / oz $ / lb
Silver 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,282
Average price per ounce/pound (in USD per Ounce) | $ / oz	25.00
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 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,858
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 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	23,314
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.19
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 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	4,905
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.90
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
Zinc 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	41,557
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.17
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 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	30,876
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.88
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
Zinc 2022 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	18,519
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.28