Note 13 - Preferred Stock and Common Stock (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended		12 Months Ended
	Jan. 31, 2018	Jun. 30, 2018		Dec. 31, 2017	Feb. 22, 2018
Preferred Stock, Par or Stated Value Per Share		$ 1		$ 1
Common Stock, Par or Stated Value Per Share		$ 0.01		$ 0.01	$ 0.01
Stock Repurchase Program, Authorized Amount					$ 300,000
Stock Repurchased During Period, Shares		5,100,000
Stock Repurchased During Period, Value		$ 75,126
Stock Repurchased During Period, Average Cost Per Share		$ 14.72
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 224,900
Series M Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues	1,380,000
Preferred stock, Fractional Interest in Share of Stock	0.001%
Preferred Stock, Dividend Rate, Percentage		5.25%	[1]	5.25%
Preferred Stock, Par or Stated Value Per Share	$ 1	$ 1	[1]	$ 1
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 33,400
Series M Cumulative Redeemable Preferred Stock [Member]
Preferred Stock, Dividend Rate, Percentage	5.25%

[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.