Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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	45	2,556
Average price per ounce/pound (in USD per Ounce) | $ / oz	17.82	17.20
Zinc 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	21,330	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.91	1.04
Lead 2020 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	7,441	5,159
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.78	0.88
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
Silver 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	2,840	5,700
Average price per ounce/pound (in USD per Ounce) | $ / oz	16.00	15.73
Gold 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	95	130
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,482	1,435
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	5,842	11,740
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.98	0.98
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