Note 12 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Receivables from provisional concentrate sales	$ 9,188	$ 6,421
Fair Value, Recurring [Member]
Total assets	236,560	82,820
Total liabilities	19,540	7,214
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	215,715	62,452
Equity securities – mining industry	4,919	6,207
Certificates of deposit and other bank deposits	1,053	1,025
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Receivables from provisional concentrate sales	6,062	11,952
Metal forward and put option contracts	8,811	0
Foreign exchange contracts	0	1,184
Metal forward and put option contracts	69	5,777
Foreign exchange contracts	$ 19,471	$ 1,437

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 of financial and nonfinancial obligations.

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

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