Tax Treatment of Distributions (Details 1) - $ / shares		1 Months Ended		3 Months Ended	12 Months Ended
		Aug. 31, 2012	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Income Tax Treatment of Dividends Paid [Line Items]
Capital gains, federal statutory tax rate							20.00%		25.00%
Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share				$ 1.400	$ 1.400		$ 1.400		$ 1.400
Common Stock, Dividends, Per Share, Cash Paid, Percentage					100.00%		100.00%		100.00%
Series G Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred stock, dividend rate			6.875%		6.875%				6.875%
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.719		$ 1.719		$ 1.719
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					100.00%		100.00%		100.00%
Series H Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred stock, dividend rate		6.375%			6.375%				6.375%
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.594		$ 1.594		$ 1.594
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					100.00%		100.00%		100.00%
Series E Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.818
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		100.00%
Series F Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.786
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		100.00%
Tax Treatment, Ordinary Income | Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share					$ 0.992		$ 0.998		$ 0.756
Common Stock, Dividends, Per Share, Cash Paid, Percentage					70.86%		71.29%		54.00%
Tax Treatment, Ordinary Income | Series G Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.218		$ 1.711		$ 1.668
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					70.86%		99.54%		97.03%
Tax Treatment, Ordinary Income | Series H Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 1.129		$ 1.587		$ 1.546
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					70.86%		99.56%		96.99%
Tax Treatment, Ordinary Income | Series E Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.818
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		100.00%
Tax Treatment, Ordinary Income | Series F Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.786
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		100.00%
Tax Treatment, Qualified Dividend | Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share					$ 0.002		$ 0.002		$ 0.003
Common Stock, Dividends, Per Share, Cash Paid, Percentage					0.13%		0.14%		0.21%
Tax Treatment, Qualified Dividend | Series G Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.002		$ 0.003		$ 0.006
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.13%		0.17%		0.35%
Tax Treatment, Qualified Dividend | Series H Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.002		$ 0.003		$ 0.006
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.13%		0.19%		0.38%
Tax Treatment, Return of Capital | Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share					$ 0.000		$ 0.398		$ 0.620
Common Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		28.43%		44.29%
Tax Treatment, Capital Gains | Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share					$ 0.051		$ 0.002	[1]	$ 0.000	[1]
Common Stock, Dividends, Per Share, Cash Paid, Percentage	[1]				3.65%		0.14%		0.00%
Tax Treatment, Capital Gains | Series G Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.063		$ 0.005	[2]	$ 0.000	[2]
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage	[2]				3.65%		0.29%		0.00%
Tax Treatment, Capital Gains | Series H Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.059		$ 0.004		$ 0.000
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					3.65%	[2]	0.25%		0.00%
Tax Treatment, Capital Gains | Series E Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.000
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		0.00%
Tax Treatment, Capital Gains | Series F Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.000
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		0.00%
Tax Treatment, Unrecaptured Section 1250 Gains | Common Stock
Income Tax Treatment of Dividends Paid [Line Items]
Dividends paid per common share					$ 0.355		$ 0.000		$ 0.021
Common Stock, Dividends, Per Share, Cash Paid, Percentage					25.36%		0.00%		1.50%
Tax Treatment, Unrecaptured Section 1250 Gains | Series G Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.436		$ 0.000		$ 0.045
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					25.36%		0.00%		2.62%
Tax Treatment, Unrecaptured Section 1250 Gains | Series H Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.404		$ 0.000		$ 0.042
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					25.36%		0.00%		2.63%
Tax Treatment, Unrecaptured Section 1250 Gains | Series E Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.000
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		0.00%
Tax Treatment, Unrecaptured Section 1250 Gains | Series F Cumulative Redeemable Preferred Stock
Income Tax Treatment of Dividends Paid [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid					$ 0.000		$ 0.000		$ 0.000
Preferred Stock, Dividends, Per Share, Cash Paid, Percentage					0.00%		0.00%		0.00%

[1] Capital gains are comprised entirely of 20% rate gains.

[2] Capital gains are comprised entirely of 20%