Note 12 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Trade accounts receivable:
Receivables from provisional concentrate sales	$ 8,925	$ 7,003
Fair Value, Measurements, Recurring [Member]
Restricted cash balances:
Certificates of deposit and other deposits	1,005	1,032
Total assets	278,326	248,206
Liabilities:
Total liabilities	2,330	15,531
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents:
Money market funds and other bank deposits	239,722	186,107
Available for sale securities:
Equity securities – mining industry	7,449	7,561
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Available for sale securities:
Debt securities - municipal and corporate bonds	5,556	33,758
Trade accounts receivable:
Receivables from provisional concentrate sales	9,717	14,805
Restricted cash balances:
Foreign exchange contracts	172	4,943
Metal forward contracts	14,705
Liabilities:
Foreign exchange contracts	2,330
Metal forward contracts		$ 15,531

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value of financial and nonfinancial obligations.

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