Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 oz lb $ / oz $ / lb	Dec. 31, 2017 oz lb $ / oz $ / lb
Silver 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	543	1,447
Average price per ounce/pound (in USD per Ounce) | $ / oz	16.91	16.64
Gold 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | oz	2	5
Average price per ounce/pound (in USD per Ounce) | $ / oz	1,342	1,279
Zinc 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb	7,385	21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.52	1.45
Lead 2018 Settlements for Provisional Sales [Member]
Ounces/pounds under contract (Ounce) | lb		4,740
Average price per ounce/pound (in USD per Ounce) | $ / lb		1.11
Zinc 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	26,841	32,187
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.23	1.29
Lead 2018 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	15,598	16,645
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.07	1.06
Zinc 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	48,502	23,589
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.4	1.33
Lead 2019 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	20,283	18,078
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.1	1.09
Zinc 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	42,329	3,307
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.4	1.27
Lead 2020 Settlements for Forecasted Sales [Member]
Ounces/pounds under contract (Ounce) | lb	19,401	2,866
Average price per ounce/pound (in USD per Ounce) | $ / lb	1.13	1.08