Tax Treatment of Distributions (Details 1) - $ / shares	1 Months Ended		12 Months Ended
	Aug. 31, 2012	Mar. 31, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 1.587
Capital gains, federal statutory tax rate			20.00%
Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 1.450	$ 1.400	$ 1.400
Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Preferred stock, dividend rate		6.875%	6.875%	6.875%
Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Preferred stock, dividend rate	6.375%		6.375%	6.375%
Tax treatment, Ordinary income [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 1.500	$ 0.992	$ 0.998
Dividends paid per common share, percentage			49.40%	70.86%	71.29%
Tax treatment, Ordinary income [Member] | Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.848	$ 1.218	$ 1.711
Dividend paid per preferred share, percentage			49.31%	70.86%	99.54%
Tax treatment, Ordinary income [Member] | Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.786	$ 1.129	$ 1.587
Dividend paid per preferred share, percentage			49.31%	70.86%	99.56%
Tax treatment, Qualified dividend [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 0.002	$ 0.002	$ 0.002
Dividends paid per common share, percentage			0.06%	0.13%	0.14%
Tax treatment, Qualified dividend [Member] | Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.001	$ 0.002	$ 0.003
Dividend paid per preferred share, percentage			0.06%	0.13%	0.17%
Tax treatment, Qualified dividend [Member] | Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.001	$ 0.002	$ 0.003
Dividend paid per preferred share, percentage			0.06%	0.13%	0.19%
Tax treatment, Return of capital [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 0.000	$ 0.000	$ 0.398
Dividends paid per common share, percentage			0.00%	0.00%	28.43%
Tax treatment, Capital gains [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 1.212	$ 0.051	$ 0.002
Dividends paid per common share, percentage			39.89%	3.65%	0.14%
Tax treatment, Capital gains [Member] | Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.687	$ 0.063	$ 0.005
Dividend paid per preferred share, percentage			39.97%	3.65%	0.29%
Tax treatment, Capital gains [Member] | Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.637	$ 0.059	$ 0.004
Dividend paid per preferred share, percentage			39.97%	3.65%	0.25%
Tax treatment, Unrecaptured section 1250 gains [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 0.323	$ 0.355	$ 0.000
Dividends paid per common share, percentage			10.65%	25.36%	0.00%
Tax treatment, Unrecaptured section 1250 gains [Member] | Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.183	$ 0.436	$ 0.000
Dividend paid per preferred share, percentage			10.66%	25.36%	0.00%
Tax treatment, Unrecaptured section 1250 gains [Member] | Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 0.170	$ 0.404	$ 0.000
Dividend paid per preferred share, percentage			10.66%	25.36%	0.00%
Tax treatment [Member] | Common stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share			$ 3.037	$ 1.400	$ 1.400
Dividends paid per common share, percentage			100.00%	100.00%	100.00%
Tax treatment [Member] | Series G Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 1.719	$ 1.719	$ 1.719
Dividend paid per preferred share, percentage			100.00%	100.00%	100.00%
Tax treatment [Member] | Series H Preferred Stock [Member]
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per preferred share			$ 1.594	$ 1.594	$ 1.594
Dividend paid per preferred share, percentage			100.00%	100.00%	100.00%