Note 8 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	6 Months Ended	12 Months Ended
	Jun. 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	2,344	1,282
Average price per ounce/pound (in USD per Ounce) | $ / oz	26.76	25.00
Gold 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	7	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,810	1,858
Zinc 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	17,857	23,314
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.32	1.19
Lead 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	11,762	4,905
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.00	0.90
Zinc 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	15,708	41,577
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.24	1.17
Lead 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	14,991	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	66,855	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	50,982
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.96
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	52,249
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	15,047
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.33