Note 9 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 oz lb $ / oz $ / lb	Dec. 31, 2020 oz lb $ / oz $ / lb
Silver 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,190	1,282
Average price per ounce/pound (in USD per Ounce) | $ / oz	23.15	25.00
Gold 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	4	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,776	1,858
Zinc 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	20,867	23,314
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.35	1.19
Lead 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	9,866	4,905
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.01	0.90
Zinc 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	7,771	41,577
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.26	1.17
Lead 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	6,779	30,876
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.94	0.88
Zinc 2022 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	60,043	18,519
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.28	1.28
Lead 2022 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	63,769
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.98
Zinc 2023 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	76,280
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.29
Lead 2023 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	70,327
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.00
Zinc 2024 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	43,762
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.31