Note 15 - Stockholders' Equity - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Shares authorized (in shares)	7,054,000	7,054,000
Liquidation preference	$ 714.5	$ 1,064.5
Par value (in dollars per share)	$ 1.00	$ 1.00
Series J Preferred Stock [Member]
Shares authorized (in shares)	9,000	9,000
Shares issued and outstanding (in shares)	9,000	9,000
Liquidation preference	$ 225.0	$ 225.0
Dividend rate	5.50%	5.50%
Annual dividend per depository share (in dollars per share)	$ 1.37500	$ 1.37500
Par value (in dollars per share)	$ 1.00	$ 1.00
Series I Preferred Stock [Member]
Shares authorized (in shares)		18,400
Shares issued and outstanding (in shares)		7,000
Liquidation preference		$ 175.0
Dividend rate		6.00%
Annual dividend per depository share (in dollars per share)		$ 1.50000
Par value (in dollars per share)		$ 1.00
Series L Preferred Stock [Member]
Shares authorized (in shares)	10,350	10,350
Shares issued and outstanding (in shares)	9,000	9,000
Liquidation preference	$ 225.0	$ 225.0
Dividend rate	5.125%	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125	$ 1.28125
Par value (in dollars per share)	$ 1.00	$ 1.00
Series M Preferred Stock [Member]
Shares authorized (in shares)	10,580	10,580
Shares issued and outstanding (in shares)	10,580	10,580
Liquidation preference	$ 264.5	$ 264.5
Dividend rate	5.25%	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.31250	$ 1.31250
Par value (in dollars per share)	$ 1.00	$ 1.00
Series K Preferred Stock [Member]
Shares authorized (in shares)		8,050
Shares issued and outstanding (in shares)		7,000
Liquidation preference		$ 175.0
Dividend rate		5.625%
Annual dividend per depository share (in dollars per share)		$ 1.40625
Par value (in dollars per share)		$ 1.00
Total [Member]
Shares issued and outstanding (in shares)	28,580	42,580
Liquidation preference	$ 714.5	$ 1,064.5