Note 21 - Income Taxes - Taxable Characteristics of Distributions Paid (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividends distributed, amount	$ 528,606	$ 526,912	$ 503,793
Series I Preferred Stock [Member]
Ordinary income, amount	$ 7,389	$ 5,565	$ 21,636
Ordinary income, percent	77.00%	53.00%	96.00%
Capital gain, amount	$ 2,207	$ 4,935	$ 902
Capital gain, percent	23.00%	47.00%	4.00%
Dividends distributed, amount	$ 9,596	$ 10,500	$ 22,538
Dividends distributed, percent	100.00%	100.00%	100.00%
Series J Preferred Stock [Member]
Ordinary income, amount	$ 11,541	$ 6,559	$ 11,880
Ordinary income, percent	77.00%	53.00%	96.00%
Capital gain, amount	$ 3,447	$ 5,816	$ 495
Capital gain, percent	23.00%	47.00%	4.00%
Dividends distributed, amount	$ 14,988	$ 12,375	$ 12,375
Dividends distributed, percent	100.00%	100.00%	100.00%
Series K Preferred Stock [Member]
Ordinary income, amount	$ 6,927	$ 5,217	$ 9,450
Ordinary income, percent	77.00%	53.00%	96.00%
Capital gain, amount	$ 2,069	$ 4,627	$ 394
Capital gain, percent	23.00%	47.00%	4.00%
Dividends distributed, amount	$ 8,996	$ 9,844	$ 9,844
Dividends distributed, percent	100.00%	100.00%	100.00%
Series L Preferred Stock [Member]
Ordinary income, amount	$ 8,879	$ 6,111	$ 1,814
Ordinary income, percent	77.00%	53.00%	96.00%
Capital gain, amount	$ 2,652	$ 5,420	$ 76
Capital gain, percent	23.00%	47.00%	4.00%
Dividends distributed, amount	$ 11,531	$ 11,531	$ 1,890
Dividends distributed, percent	100.00%	100.00%	100.00%
Series M Preferred Stock [Member]
Ordinary income, amount	$ 10,692	$ 6,031	$ 0
Ordinary income, percent	77.00%	53.00%	0.00%
Capital gain, amount	$ 3,194	$ 5,348	$ 0
Capital gain, percent	23.00%	47.00%	0.00%
Dividends distributed, amount	$ 13,886	$ 11,379	$ 0
Dividends distributed, percent	100.00%	100.00%	0.00%
Common Stock 1 [Member]
Ordinary income, amount	$ 328,726	$ 235,642	$ 260,573
Ordinary income, percent	70.00%	50.00%	57.00%
Capital gain, amount	$ 98,618	$ 212,077	$ 9,143
Capital gain, percent	21.00%	45.00%	2.00%
Dividends distributed, amount	$ 469,609	$ 471,283	$ 457,146
Dividends distributed, percent	100.00%	100.00%	100.00%
Return of capital, amount	$ 42,265	$ 23,564	$ 187,430
Return of capital, percent	9.00%	5.00%	41.00%