Note 18 - Preferred Stock, Common Stock and Convertible Unit Transactions - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Preferred stock, shares authorized (in shares)	7,054,000	7,054,000
Preferred stock, liquidation preference	$ 489.5	$ 489.5
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Series L Preferred Stock [Member]
Preferred stock, shares authorized (in shares)	10,350
Shares issued and outstanding (in shares)	9,000
Preferred stock, liquidation preference	$ 225.0
Dividend rate	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125
Preferred stock, par value (in dollars per share)	$ 1.00
Series M Preferred Stock [Member]
Preferred stock, shares authorized (in shares)	10,580
Shares issued and outstanding (in shares)	10,580
Preferred stock, liquidation preference	$ 264.5
Dividend rate	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.31250
Preferred stock, par value (in dollars per share)	$ 1.00
Redeemable Preferred Stock [Member]
Shares issued and outstanding (in shares)	19,580
Preferred stock, liquidation preference	$ 489.5