Beneficiaries Equity of The Parent Company (DetailsTextuals) (USD $)	3 Months Ended			3 Months Ended	0 Months Ended	3 Months Ended	0 Months Ended			3 Months Ended		25 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	May 02, 2006	Mar. 31, 2012 7.50% Series C Preferred Shares	Apr. 16, 2012 7.50% Series C Preferred Shares Dividend Paid	Mar. 31, 2012 7.375% Series D Preferred Shares	Apr. 16, 2012 7.375% Series D Preferred Shares Dividend Paid	Apr. 19, 2012 7.375% Series D Preferred Shares Dividend Paid	Apr. 05, 2012 7.375% Series D Preferred Shares Dividend Paid	Mar. 31, 2012 Offering Program [Member]	Mar. 31, 2011 Offering Program [Member]	Mar. 31, 2012 Offering Program [Member]
Class of Stock [Line Items]
Percentage of Preferential return				7.50%		7.375%
Dividends paid for preferred shares	$ 1,998,000	$ 1,998,000			$ 900,000		$ 1,100,000
Beneficiaries Equity of The Parent Company (Textuals) [Abstract]
Antidilutive securities excluded from computation of earnings per share	2,657,721	9,902,752
Distribution Declared per common partnership unit								$ 0.15
Amount of dividend declared									21,600,000
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											188,400
Average sales price per share											$ 12.23	$ 12.50
Issuance of Common Shares of Beneficial Interest		$ 2,305,000									$ 2,200,000
Common partnership units issued to the parent company related to redemption											188,400
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