Note 14 - Stockholders' Equity - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Shares authorized (in shares)	7,054,000	7,054,000
Liquidation preference	$ 489.5	$ 489.5
Par value (in dollars per share)	$ 1.00	$ 1.00
Series L Preferred Stock [Member]
Shares authorized (in shares)	10,350
Shares issued and outstanding (in shares)	9,000
Liquidation preference	$ 225.0
Dividend rate	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125
Par value (in dollars per share)	$ 1.00
Series M Preferred Stock [Member]
Shares authorized (in shares)	10,580
Shares issued and outstanding (in shares)	10,580
Liquidation preference	$ 264.5
Dividend rate	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.31250
Par value (in dollars per share)	$ 1.00
Total [Member]
Shares issued and outstanding (in shares)	19,580
Liquidation preference	$ 489.5