Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017 oz lb $ / oz $ / lb	Dec. 31, 2016 oz lb $ / oz $ / lb
Silver 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	1,379	1,295
Average price per ounce/pound (in USD per Ounce) | $ / oz	17.14	16.29
Gold 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	7	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,259	1,172
Zinc 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	15,873	19,070
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.21	1.18
Lead 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	6,724	7,441
Average price per ounce/pound (in USD per Ounce) | $ / lb	0.97	0.97
Zinc 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	6,834	35,384
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.26	1.19
Lead 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	6,504	17,637
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.05	1.03
Zinc 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	28,329	13,779
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.23	1.21
Lead 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	16,314	5,732
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.05	1.05
Zinc 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	1,102
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.21
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	1,102
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.06