Note 8 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 oz lb g $ / oz $ / lb $ / item	Dec. 31, 2020 oz lb $ / oz $ / lb
Silver 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,492	1,282
Average price per ounce/pound (in USD per Ounce) | $ / oz	25.48	25.00
Gold 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	5	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,736	1,858
Zinc 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	12,070	23,314
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.26	1.19
Lead 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	1,587	4,905
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.89	0.90
Zinc 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	33,841	41,577
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.20	1.17
Lead 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	30,479	30,876
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.89	0.88
Zinc 2022 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	53,407	18,519
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.26	1.28
Lead 2022 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | g	42,715
Average price per ounce/pound (in USD per Ounce) | $ / item	0.96
Zinc 2023 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	41,171
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.27