Note 9 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Receivables from provisional concentrate sales	$ 9,334	$ 11,329
Fair Value, Recurring [Member]
Total assets	245,671	186,164
Total liabilities	13,860	11,756
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	181,494	129,830
Equity securities	11,083	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	41,311	27,864
Foreign exchange contracts	10,341	7,647
Metal forward and put option contracts	389	381
Foreign exchange contracts	0	19
Metal forward and put option contracts	$ 13,860	$ 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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A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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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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