Note 6 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Feb. 21, 2022	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Share-Based Payment Arrangement, Expense		$ 1.3		$ 0.5
Common Stock, Dividends, Per Share, Declared (in dollars per share)		$ 0.00625		$ 0.00875
Average Realized Silver Price Per Ounce (in dollars per share)			$ 23.49
Quarterly Dividends [Member]
Common Stock, Dividends, Per Share, Declared (in dollars per share)	$ 0.00625
Quarterly Dividends, Common Stock, Cash, Minimum Dividend Component (in dollars per share)	0.00375
Quarterly Dividends, Common Stock, Cash, Silver Price-linked Component (in dollars per share)	$ 0.0025
Dividends, Common Stock, Total		$ 3.4
Dividends Payable, Date Declared	Feb. 21, 2022