Note 6 - Stockholders' Equity - Common Stock Dividend Policy (Details)	6 Months Ended
	Jun. 30, 2021 $ / shares
Quarterly Average Realized Price, Level 1 [Member]
Quarterly dividend per share (in dollars per share)	$ 0.75
Annual dividend per share (in dollars per share)	3
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	4.5
Quarterly Average Realized Price, Level 2 [Member]
Quarterly dividend per share (in dollars per share)	1.25
Annual dividend per share (in dollars per share)	5
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	6.5
Quarterly Average Realized Price, Level 3 [Member]
Quarterly dividend per share (in dollars per share)	2.25
Annual dividend per share (in dollars per share)	9
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	10.5
Quarterly Average Realized Price, Level 4 [Member]
Quarterly dividend per share (in dollars per share)	3.25
Annual dividend per share (in dollars per share)	13
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	14.5
Quarterly Average Realized Price, Level 5 [Member]
Quarterly dividend per share (in dollars per share)	4.25
Annual dividend per share (in dollars per share)	17
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	18.5
Quarterly Average Realized Price, Level 6 [Member]
Quarterly dividend per share (in dollars per share)	5.25
Annual dividend per share (in dollars per share)	21
Minimum annual component per share (in dollars per share)	1.5
Annualized dividends (in dollars per share)	$ 22.5