Note 12 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Trade accounts receivable:
Receivables from provisional concentrate sales	$ 7,003	$ 9,780
Fair Value, Measurements, Recurring [Member]
Restricted cash balances:
Total assets	248,206	231,608
Liabilities
Total liabilities	15,531	5,480
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents:
Money market funds and other bank deposits	186,107	169,777
Available for sale securities:
Equity securities – mining industry	7,561	5,002
Restricted cash balances:
Certificates of deposit and other bank deposits	1,032	2,200
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Available for sale securities:
Debt securities – municipal and corporate bonds	33,758	29,117
Trade accounts receivable:
Receivables from provisional concentrate sales	14,805	20,082
Derivative contracts:
Metal forward contracts		5,403
Foreign exchange contracts	4,943	27
Liabilities
Metal forward contracts	15,531	192
Foreign exchange contracts		$ 5,288

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