Note 13 - Preferred Stock and Common Stock - Outstanding Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 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, 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		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
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
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
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
Series M Preferred Stock [Member]
Shares authorized (in shares)	10,580	[1]	10,580
Shares issued and outstanding (in shares)	10,580	[1]	9,200
Liquidation preference	$ 264,500	[1]	$ 230,000
Preferred Stock, Dividend Rate, Percentage	5.25%	[1]	5.25%
Annual dividend per depository share (in dollars per share)	$ 1.3125	[1]	$ 1.3125
Preferred Stock, Par or Stated Value Per Share	$ 1	[1]	$ 1	$ 1
Total [Member]
Shares issued and outstanding (in shares)	42,580		41,200
Liquidation preference	$ 1,064,500		$ 1,030,000

[1] During January 2018, the underwriting financial institutions for the Class M Preferred Stock issuance elected to exercise the over-allotment option and as a result, the Company issued an additional 1,380,000 Class M Depositary Shares, each representing a one-thousandth fractional interest in a share of the Company's 5.250% Class M Cumulative Redeemable Preferred Stock, $1.00 par value per share. The Company received net proceeds before expenses of $33.4 million from this over-allotment issuance.