Beneficiaries Equity of The Parent Company (DetailsTextuals) (USD $)	3 Months Ended		6 Months Ended			0 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended			6 Months Ended		28 Months Ended	0 Months Ended	6 Months Ended		6 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	May 02, 2006	May 03, 2012 7.50% Series C Preferred Shares	Jun. 30, 2012 7.50% Series C Preferred Shares	Jul. 16, 2012 7.375% Series D Preferred Shares	Jun. 30, 2012 7.375% Series D Preferred Shares	Jul. 19, 2012 7.375% Series D Preferred Shares Dividend Paid	Jul. 05, 2012 7.375% Series D Preferred Shares Dividend Paid	Jun. 30, 2012 Offering Program [Member]	Jun. 30, 2011 Offering Program [Member]	Jun. 30, 2012 Offering Program [Member]	Jul. 16, 2012 6.90% Series E Preferred Stock [Member]	Jun. 30, 2012 6.90% Series E Preferred Stock [Member]	Apr. 11, 2012 6.90% Series E Preferred Stock [Member]	Jun. 30, 2012 6.90% Series E Preferred Stock [Member]	Jun. 30, 2012 Subsidiaries [Member]	Jun. 30, 2011 Subsidiaries [Member]	Jun. 30, 2012 Subsidiaries [Member]	Jun. 30, 2011 Subsidiaries [Member]	Dec. 31, 2011 Subsidiaries [Member]	Jul. 19, 2012 Subsidiaries [Member]	Jul. 05, 2012 Subsidiaries [Member]	Jun. 30, 2012 Subsidiaries [Member] 7.50% Series C Preferred Shares	Jun. 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			$ (47,910,000)			$ 50,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock			96,240,000	0												96,300,000					96,240,000	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			5,047,000	3,996,000			200,000	1,100,000							1,800,000
Redemption Premium	2,090,000	0	2,090,000	0															2,090,000	0	2,090,000	0				(2,100,000)
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													$ 12.18	$ 12.50
Issuance of Common Shares of Beneficial Interest				8,272,000									8,000,000
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											$ 21,600,000														$ 21,600,000