Note 15 - Stockholders' Equity - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Shares authorized (in shares)	5,996,240	5,996,240
Liquidation preference	$ 1,064,500	$ 1,064,500
Par value (in dollars per share)	$ 1	$ 1
Series I Preferred Stock [Member]
Shares authorized (in shares)	18,400
Shares issued and outstanding (in shares)	7,000
Liquidation preference	$ 175,000
Dividend rate	6.00%
Annual dividend per depository share (in dollars per share)	$ 1.50
Par value (in dollars per share)	$ 1
Series J Preferred Stock [Member]
Shares authorized (in shares)	9,000
Shares issued and outstanding (in shares)	9,000
Liquidation preference	$ 225,000
Dividend rate	5.50%
Annual dividend per depository share (in dollars per share)	$ 1.375
Par value (in dollars per share)	$ 1
Series K Preferred Stock [Member]
Shares authorized (in shares)	8,050
Shares issued and outstanding (in shares)	7,000
Liquidation preference	$ 175,000
Dividend rate	5.625%
Annual dividend per depository share (in dollars per share)	$ 1.40625
Par value (in dollars per share)	$ 1
Series L Preferred Stock [Member]
Shares authorized (in shares)	10,350
Shares issued and outstanding (in shares)	9,000
Liquidation preference	$ 225,000
Dividend rate	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125
Par value (in dollars per share)	$ 1
Series M Preferred Stock [Member]
Shares authorized (in shares)	10,580
Shares issued and outstanding (in shares)	10,580
Liquidation preference	$ 264,500
Dividend rate	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.3125
Par value (in dollars per share)	$ 1
Total [Member]
Shares issued and outstanding (in shares)	42,580
Liquidation preference	$ 1,064,500