Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	9 Months Ended	12 Months Ended
	Sep. 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	1,399	1,295
Average price per ounce/pound (in USD per Ounce) | $ / oz	17.18	16.29
Gold 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	5	4
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,298	1,172
Zinc 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	19,070	19,070
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.33	1.18
Lead 2017 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	2,535	7,441
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.07	0.97
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	2,370
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.38
Zinc 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	441	35,384
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.23	1.19
Lead 2017 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	2,866	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	39,463	13,779
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.27	1.21
Lead 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	17,968	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	14,330
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.3
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	8,267
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.07
Silver 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | oz
Gold 2020 Settlements for Forecastedl Sales [Member]
Ounces/pounds under contract (Ounce) | oz
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	3,307
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.27
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	2,205
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.07