Note 13 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Receivables from provisional concentrate sales	$ 7,612	$ 7,003
Fair Value, Measurements, Recurring [Member]
Certificates of deposit and other bank deposits	1,010	1,032
Total assets	84,045	248,206
Total liabilities	740	15,531
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Money market funds and other bank deposits	60,856	186,107
Equity securities – mining industry	7,190	7,561
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Debt securities - municipal and corporate bonds		33,758
Receivables from provisional concentrate sales	12,947	14,805
Foreign exchange contracts	1,760	4,943
Metal forward contracts	282
Foreign exchange contracts	136
Metal forward contracts	$ 604	$ 15,531

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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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts.

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Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

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Fair value of financial and nonfinancial obligations.

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Amount due from parties in nontrade transactions, classified as other.

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