Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 oz $ / oz	Dec. 31, 2018 oz $ / oz
Silver 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	710	842
Average price per ounce/pound (in USD per Ounce) | $ / oz	15.46	14.69
Gold 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	3	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,316	1,260
Zinc 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	19,952
Average price per ounce/pound (in USD per Ounce) | $ / oz	1.22
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz		18,450
Average price per ounce/pound (in USD per Ounce) | $ / oz		1.15
Lead 2019 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	2,646	2,700
Average price per ounce/pound (in USD per Ounce) | $ / oz	0.92	0.89
Zinc 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	26,180
Average price per ounce/pound (in USD per Ounce) | $ / oz	1.25
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,653
Average price per ounce/pound (in USD per Ounce) | $ / oz	0.96
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	276
Average price per ounce/pound (in USD per Ounce) | $ / oz	1.26
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz	551
Average price per ounce/pound (in USD per Ounce) | $ / oz	0.96