Note 6 - Stockholders' Equity - Common Stock Dividend Policy (Details)	3 Months Ended
	Mar. 31, 2021 $ / shares
Quarterly Average Realized Price, Level 1 [Member]
Quarterly dividend per share (in dollars per share)	$ 0.0075
Annual dividend per share (in dollars per share)	0.03
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	0.045
Quarterly Average Realized Price, Level 2 [Member]
Quarterly dividend per share (in dollars per share)	0.0125
Annual dividend per share (in dollars per share)	0.05
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	0.065
Quarterly Average Realized Price, Level 3 [Member]
Quarterly dividend per share (in dollars per share)	0.0225
Annual dividend per share (in dollars per share)	0.09
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	0.105
Quarterly Average Realized Price, Level 4 [Member]
Quarterly dividend per share (in dollars per share)	0.0325
Annual dividend per share (in dollars per share)	0.13
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	0.145
Quarterly Average Realized Price, Level 5 [Member]
Quarterly dividend per share (in dollars per share)	0.0425
Annual dividend per share (in dollars per share)	0.17
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	0.185
Quarterly Average Realized Price, Level 6 [Member]
Quarterly dividend per share (in dollars per share)	0.0525
Annual dividend per share (in dollars per share)	0.21
Minimum annual component per share (in dollars per share)	0.015
Annualized dividends (in dollars per share)	$ 0.225