Note 21 - Income Taxes - Taxable Characteristics of Distributions Paid (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends distributed, amount	$ 526,912	$ 503,793	$ 471,850
Dividends distributed, percent
Series I Preferred Stock [Member]
Ordinary income, amount	$ 5,565	$ 21,636	$ 16,320
Ordinary income, percent	53.00%	96.00%	68.00%
Capital gain, amount	$ 4,935	$ 902	$ 7,680
Capital gain, percent	47.00%	4.00%	32.00%
Dividends distributed, amount	$ 10,500	$ 22,538	$ 24,000
Dividends distributed, percent	100.00%	100.00%	100.00%
Series J Preferred Stock [Member]
Ordinary income, amount	$ 6,559	$ 11,880	$ 8,415
Ordinary income, percent	53.00%	96.00%	68.00%
Capital gain, amount	$ 5,816	$ 495	$ 3,960
Capital gain, percent	47.00%	4.00%	32.00%
Dividends distributed, amount	$ 12,375	$ 12,375	$ 12,375
Dividends distributed, percent	100.00%	100.00%	100.00%
Series K Preferred Stock [Member]
Ordinary income, amount	$ 5,217	$ 9,450	$ 6,694
Ordinary income, percent	53.00%	96.00%	68.00%
Capital gain, amount	$ 4,627	$ 394	$ 3,150
Capital gain, percent	47.00%	4.00%	32.00%
Dividends distributed, amount	$ 9,844	$ 9,844	$ 9,844
Dividends distributed, percent	100.00%	100.00%	100.00%
Series L Preferred Stock [Member]
Ordinary income, amount	$ 6,111	$ 1,814
Ordinary income, percent	53.00%	96.00%
Capital gain, amount	$ 5,420	$ 76
Capital gain, percent	47.00%	4.00%
Dividends distributed, amount	$ 11,531	$ 1,890
Dividends distributed, percent	100.00%	100.00%
Series M Preferred Stock [Member]
Ordinary income, amount	$ 6,031
Ordinary income, percent	53.00%
Capital gain, amount	$ 5,348
Capital gain, percent	47.00%
Dividends distributed, amount	$ 11,379
Dividends distributed, percent	100.00%
Common Stock 1 [Member]
Ordinary income, amount	$ 235,642	$ 260,573	$ 263,892
Ordinary income, percent	50.00%	57.00%	62.00%
Capital gain, amount	$ 212,077	$ 9,143	$ 127,689
Capital gain, percent	45.00%	2.00%	30.00%
Dividends distributed, amount	$ 471,283	$ 457,146	$ 425,631
Dividends distributed, percent	100.00%	100.00%	100.00%
Return of capital, amount	$ 23,564	$ 187,430	$ 34,050
Return of capital, percent	5.00%	41.00%	8.00%