Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 oz lb $ / oz $ / lb	Dec. 31, 2019 oz lb $ / oz $ / lb
Silver 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	910	2,556
Average price per ounce/pound (in USD per Ounce) | $ / oz	23.53	17.20
Gold 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	2	10
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,948	1,481
Zinc 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	23,975	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.02	1.04
Lead 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	5,512	5,159
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.84	0.88
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	827	441
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.94	1.13
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	8,543	11,740
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.81	0.98
Silver 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Gold 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Zinc 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	6,945
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.12
Lead 2021 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	0
Silver 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Gold 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Silver 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Gold 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	0
Zinc 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	27,448
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.06
Lead 2021 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	12,136
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.86