Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 oz lb $ / oz $ / lb	Dec. 31, 2019 oz lb $ / oz $ / lb
Silver 2019 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | oz	2,303
2020 settlements (in USD per Ounce) | $ / oz	16.97
Gold 2019 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | oz	7
2020 settlements (in USD per Ounce) | $ / oz	1,728
Zinc 2019 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | lb	17,086
2020 settlements (in USD per Ounce) | $ / lb	0.89
Lead 2019 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | lb	11,409
2020 settlements (in USD per Ounce) | $ / lb	0.75
Silver 2020 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | oz		2,556
2020 settlements (in USD per Ounce) | $ / oz		17.20
Gold 2020 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | oz		10
2020 settlements (in USD per Ounce) | $ / oz		1,481
Zinc 2020 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | lb		21,550
2020 settlements (in USD per Ounce) | $ / lb		1.04
Lead 2020 Settlements for Provisional Sales [Member]
2020 settlements (Ounce) | lb		5,159
2020 settlements (in USD per Ounce) | $ / lb		0.88
Silver 2020 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | oz	2,718	5,700
2020 settlements (in USD per Ounce) | $ / oz	15.67	15.73
Gold 2020 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | oz	73	130
2020 settlements (in USD per Ounce) | $ / oz	1,633	1,435
Zinc 2019 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | lb	26,731
2020 settlements (in USD per Ounce) | $ / lb	0.88
Lead 2019 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | lb	8,322
2020 settlements (in USD per Ounce) | $ / lb	0.78
Zinc 2020 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | lb	4,134	441
2020 settlements (in USD per Ounce) | $ / lb	0.91	1.13
Lead 2020 Settlements for Forecasted Sales [Member]
2020 settlements (Ounce) | lb	1,102	11,740
2020 settlements (in USD per Ounce) | $ / lb	0.77	0.98