Note 15 - Stockholders' Equity - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Shares authorized (in shares)	5,996,240	5,996,240
Liquidation preference	$ 1,064.5	$ 1,064.5
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 J Preferred Stock [Member]
Shares authorized (in shares)	9,000
Shares issued and outstanding (in shares)	9,000
Liquidation preference	$ 225.0
Dividend rate	5.50%
Annual dividend per depository share (in dollars per share)	$ 1.37500
Par value (in dollars per share)	$ 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
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)	42,580
Liquidation preference	$ 1,064.5