Note 6 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, shares in Thousands	1 Months Ended		3 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	May 06, 2021	Feb. 18, 2021	Feb. 16, 2021
Share-based Payment Arrangement, Expense			$ 500,000		$ 1,200,000
Quarterly Dividends, Common Stock, Cash (in dollars per share)								$ 0.00875
Quarterly Dividends, Common Stock, Cash, Silver Price-linked Component (in dollars per share)								0.00375
Quarterly Dividends, Common Stock, Cash, Minimum Annual Dividend Component (in dollars per share)								$ 0.005
Dividends, Total	$ 4,700,000
Average Realized Silver Price Per Ounce				$ 25.16
Average Realized Silver Price, Minimum Dividend, Threshold (in dollars per share)	$ 25.66	$ 25.66	$ 25.66
At-the-market Offering [Member]
Equity Distribution Agreement, Maximum Number of Shares to be Sold (in shares)							60,000
Stock Issued During Period, Shares, New Issues (in shares)		0
Subsequent Event [Member]
Quarterly Dividends, Common Stock, Cash (in dollars per share)						$ 0.01
Quarterly Dividends, Common Stock, Cash, Silver Price-linked Component (in dollars per share)						$ 0.075