Note 8 - Shareholders' Equity - Common Stock Dividend Policy (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares $ / item
Quarterly Average Realized Price, Level 1 [Member]
Quarterly average realized silver price per ounce (in USD per Per Share) | $ / item	30
Quarterly dividend per share (in dollars per share)	$ 0.01
Annual dividend per share (in dollars per share)	$ 0.04
Quarterly Average Realized Price, Level 2 [Member]
Quarterly average realized silver price per ounce (in USD per Per Share) | $ / item	35
Quarterly dividend per share (in dollars per share)	$ 0.02
Annual dividend per share (in dollars per share)	$ 0.08
Quarterly Average Realized Price, Level 3 [Member]
Quarterly average realized silver price per ounce (in USD per Per Share) | $ / item	40
Quarterly dividend per share (in dollars per share)	$ 0.03
Annual dividend per share (in dollars per share)	$ 0.12
Quarterly Average Realized Price, Level 4 [Member]
Quarterly average realized silver price per ounce (in USD per Per Share) | $ / item	45
Quarterly dividend per share (in dollars per share)	$ 0.04
Annual dividend per share (in dollars per share)	$ 0.16
Quarterly Average Realized Price, Level 5 [Member]
Quarterly average realized silver price per ounce (in USD per Per Share) | $ / item	50
Quarterly dividend per share (in dollars per share)	$ 0.05
Annual dividend per share (in dollars per share)	$ 0.20