Note 12 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Receivables from provisional concentrate sales	$ 5,162	$ 7,443
Fair Value, Recurring [Member]
Total assets	29,536	39,413
Total liabilities	8,706	8,968
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	11,797	27,389
Equity securities – mining industry	6,768	6,583
Certificates of deposit and other bank deposits	1,025	1,025
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Receivables from provisional concentrate sales	9,586	4,184
Metal forward contracts	207	209
Foreign exchange contracts	153	23
Metal forward contracts	4,057	373
Foreign exchange contracts	$ 4,649	$ 8,595

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

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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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