Note 10 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Receivables from provisional concentrate sales	$ 10,152	$ 11,329
Fair Value, Recurring [Member]
Total assets	244,776	186,164
Total liabilities	4,931	11,756
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	190,904	129,830
Equity securities	8,030	19,389
Certificates of deposit and other deposits	1,053	1,053
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Receivables from provisional concentrate sales	32,821	27,864
Foreign exchange contracts	4,655	7,647
Metal forward and put option contracts	7,313	381
Foreign exchange contracts	200	19
Metal forward and put option contracts	$ 4,731	$ 11,737

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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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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