Beneficiaries Equity of The Parent Company (DetailsTextuals) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended				0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended			9 Months Ended		0 Months Ended	9 Months Ended		9 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended			0 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	May 02, 2006	May 03, 2012 7.50% Series C Preferred Shares	Sep. 30, 2012 7.50% Series C Preferred Shares	Oct. 16, 2012 7.375% Series D Preferred Shares	Sep. 30, 2012 7.375% Series D Preferred Shares	Oct. 19, 2012 7.375% Series D Preferred Shares Dividend Paid	Oct. 05, 2012 7.375% Series D Preferred Shares Dividend Paid	Sep. 30, 2012 Offering Program [Member]	Sep. 30, 2011 Offering Program [Member]	Oct. 16, 2012 6.90% Series E Preferred Stock [Member]	Sep. 30, 2012 6.90% Series E Preferred Stock [Member]	Apr. 11, 2012 6.90% Series E Preferred Stock [Member]	Sep. 30, 2012 6.90% Series E Preferred Stock [Member]	Sep. 30, 2012 Subsidiaries [Member]	Sep. 30, 2011 Subsidiaries [Member]	Sep. 30, 2012 Subsidiaries [Member]	Sep. 30, 2011 Subsidiaries [Member]	Dec. 31, 2011 Subsidiaries [Member]	Oct. 19, 2012 Subsidiaries [Member]	Oct. 05, 2012 Subsidiaries [Member]	May 03, 2012 Subsidiaries [Member] 7.50% Series C Preferred Shares	Sep. 30, 2012 Subsidiaries [Member] 6.90% Series E Preferred Stock [Member]	Dec. 31, 2011 Subsidiaries [Member] 6.90% Series E Preferred Stock [Member]
Class of Stock [Line Items]
Stock Redeemed or Called During Period, Value			$ (47,910)				$ 50,000
Net Proceeds from Issuance of Preferred Stock and Preference Stock			96,240	0												96,300					96,240	0
Stock Redeemed or Called During Period, Shares							2,000,000
Percentage of Preferential return										7.375%						6.90%		6.90%			6.90%		0.00%				7.375%	7.375%
Dividends paid for preferred shares			7,832	5,994				200	1,100						1,800
Redemption Premium	0	0	2,090	0															0	0	2,090	0				(2,100)
Beneficiaries Equity of The Parent Company (Textuals) [Abstract]
Antidilutive securities excluded from computation of earnings per share			2,657,721	9,809,759
Preferred Stock, Shares Issued																	4,000,000	4,000,000	4,000,000		4,000,000		0				2,300,000	2,300,000
Distribution Declared per common partnership unit											$ 0.15													$ 0.15
Shares authorized for Equity Offering Program													15,000,000
Maximum percentage of Gross sale price per share, compensation to sales agents													2.00%
Issuance of Common Shares of Beneficial Interest, shares														679,285
Average sales price per share													$ 12.50	$ 12.18
Issuance of Common Shares of Beneficial Interest				8,272										8,000
Common partnership units issued to the parent company related to redemption														679,285
Number of shares sold by the company													6,421,553
Remaining shares available for sales													8,578,447
Common shares capacity restored						3,500,000
Additional shares Expected to be purchased under program	500,000		500,000
Preferred Stock, Liquidation Preference Per Share																$ 25	$ 25
Preferred Stock, per share per annum																$ 1.725
Dividends Payable, Amount	$ 24,820	$ 23,505	$ 24,820	$ 23,505	$ 23,895							$ 21,600							$ 24,820	$ 23,505	$ 24,820	$ 23,505	$ 23,895		$ 21,600