Note 10 - Outstanding Preferred Stock (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Series I Preferred Stock [Member]
Shares authorized (in shares)	18,400
Shares issued and outstanding (in shares)	16,000
Liquidation preference	$ 400,000,000
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	$ 225,000,000
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	$ 175,000,000
Dividend rate	5.625%
Annual dividend per depository share (in dollars per share)	$ 1.40625
Par value (in dollars per share)	$ 1
Total [Member]
Shares authorized (in shares)	35,450
Shares issued and outstanding (in shares)	32,000
Liquidation preference	$ 800,000,000
Shares authorized (in shares)	6,029,100	6,029,100
Liquidation preference	$ 800,000,000	$ 800,000,000
Par value (in dollars per share)	$ 1	$ 1