Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) lb oz	Dec. 31, 2018 USD ($) lb oz
Zinc 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Pound) | lb	15,047
Average price per ounce/pound	$ 1.19
Lead 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Pound) | lb	3,329	2,700
Average price per ounce/pound	$ 0.89	$ 0.89
Silver 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Pound) | oz		842
Average price per ounce/pound		$ 14.69
Gold 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Pound) | oz		4
Average price per ounce/pound		$ 1,260
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Pound) | lb		18,450
Average price per ounce/pound		$ 1.15
Silver Settlements For Forecasted Market Prices [Member]
Ounces/pounds under contract (Pound) | oz	3,059
Average price per ounce/pound	$ 14.56
Gold Settlements For Forecasted Market Prices [Member]
Ounces/pounds under contract (Pound) | oz	106
Average price per ounce/pound	$ 1,317
Zinc 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Pound) | lb	25,629
Average price per ounce/pound	$ 1.25
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Pound) | lb	1,653
Average price per ounce/pound	$ 0.96
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Pound) | lb	12,125
Average price per ounce/pound	$ 1.27
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Pound) | lb	1,102
Average price per ounce/pound	$ 0.96