Note 16 - Preferred Stock, Common Stock and Convertible Unit Transactions - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 31, 2018
Shares authorized (in shares)	5,996,240	5,996,240
Liquidation preference	$ 1,064,500	$ 1,030,000
Preferred Stock, Dividend Rate, Percentage
Annual dividend per depository share (in dollars per share)
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
Preferred stock, shares authorized (in shares)	5,996,240	5,996,240
Preferred stock, liquidation preference	$ 1,064,500	$ 1,030,000
Dividend rate
Annual dividend per depository share (in dollars per share)
Preferred stock, par value (in dollars 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	7,000
Liquidation preference	$ 175,000	$ 175,000
Preferred Stock, Dividend Rate, Percentage	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
Preferred stock, shares authorized (in shares)	18,400	18,400
Shares issued and outstanding (in shares)	7,000	7,000
Preferred stock, liquidation preference	$ 175,000	$ 175,000
Dividend rate	6.00%	6.00%
Annual dividend per depository share (in dollars per share)	$ 1.50	$ 1.50
Preferred stock, par value (in dollars 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	$ 225,000
Preferred Stock, Dividend Rate, Percentage	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
Preferred stock, shares authorized (in shares)	9,000	9,000
Shares issued and outstanding (in shares)	9,000	9,000
Preferred stock, liquidation preference	$ 225,000	$ 225,000
Dividend rate	5.50%	5.50%
Annual dividend per depository share (in dollars per share)	$ 1.375	$ 1.375
Preferred stock, par value (in dollars 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	$ 175,000
Preferred Stock, Dividend Rate, Percentage	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
Preferred stock, shares authorized (in shares)	8,050	8,050
Shares issued and outstanding (in shares)	7,000	7,000
Preferred stock, liquidation preference	$ 175,000	$ 175,000
Dividend rate	5.625%	5.625%
Annual dividend per depository share (in dollars per share)	$ 1.40625	$ 1.40625
Preferred stock, par value (in dollars per share)	$ 1	$ 1
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	$ 225,000	$ 225,000
Preferred Stock, Dividend Rate, Percentage	5.125%	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125	$ 1.28125
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1
Preferred stock, shares authorized (in shares)	10,350	10,350
Shares issued and outstanding (in shares)	9,000	9,000
Preferred stock, liquidation preference	$ 225,000	$ 225,000
Dividend rate	5.125%	5.125%
Annual dividend per depository share (in dollars per share)	$ 1.28125	$ 1.28125
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Series M Preferred Stock [Member]
Shares authorized (in shares)	10,580	10,580
Shares issued and outstanding (in shares)	10,580	9,200
Liquidation preference	$ 264,500	$ 230,000
Preferred Stock, Dividend Rate, Percentage	5.25%	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.3125	$ 1.3125
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1	$ 1
Preferred stock, shares authorized (in shares)	10,580	10,580
Shares issued and outstanding (in shares)	10,580	9,200
Preferred stock, liquidation preference	$ 264,500	$ 230,000
Dividend rate	5.25%	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.3125	$ 1.3125
Preferred stock, par value (in dollars per share)	$ 1	$ 1	$ 1
Total [Member]
Shares authorized (in shares)
Shares issued and outstanding (in shares)	42,580	41,200
Liquidation preference	$ 1,064,500	$ 1,030,000
Preferred stock, shares authorized (in shares)
Shares issued and outstanding (in shares)	42,580	41,200
Preferred stock, liquidation preference	$ 1,064,500	$ 1,030,000