Note 12 - Preferred Stock and Common Stock - Outstanding Preferred Stock (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Shares authorized (in shares)	6,017,400	6,029,100
Liquidation preference	$ 800,000	$ 800,000
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
Series I Preferred Stock [Member]
Shares authorized (in shares)	18,400	18,400
Shares issued and outstanding (in shares)	7,000	16,000
Liquidation preference	$ 175,000,000	$ 400,000,000
Dividend rate	6.00%	6.00%
Annual dividend per depository share (in dollars per share)	$ 1.50	$ 1.50
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
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	$ 225,000,000	$ 225,000,000
Dividend rate	5.50%	5.50%
Annual dividend per depository share (in dollars per share)	$ 1.375	$ 1.375
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
Series K Preferred Stock [Member]
Shares authorized (in shares)	8,050	8,050
Shares issued and outstanding (in shares)	7,000	7,000
Liquidation preference	$ 175,000,000	$ 175,000,000
Dividend rate	5.625%	5.625%
Annual dividend per depository share (in dollars per share)	$ 1.40625	$ 1.40625
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
Series L Preferred Stock [Member]
Shares authorized (in shares)	10,350
Shares issued and outstanding (in shares)	9,000
Liquidation preference	$ 225,000,000
Dividend rate	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125
Preferred Stock, Par or Stated Value Per Share	$ 1
Total [Member]
Shares authorized (in shares)	45,800	35,450
Shares issued and outstanding (in shares)	32,000	32,000
Liquidation preference	$ 800,000,000	$ 800,000,000