Acquisitions and Dispositions (Details 1) (USD $)	6 Months Ended			6 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Maryland [Member]	Jan. 19, 2011 Maryland [Member]	Jun. 30, 2011 Florida [Member]	Feb. 03, 2011 Florida [Member]	Jun. 30, 2011 Discontinued Operations [Member]	Dec. 31, 2010 Discontinued Operations [Member]
Asset Dispositions
Date Disposed					Jan. 19, 2011		Feb. 03, 2011
Net Proceeds	$ 3,775,000	$ 23,623,000		$ 3,400,000		$ 400,000
Net Real Estate Investment	6,600,000			3,500,000		3,100,000
Mortgage Note Receivable	(2,700,000)					(2,700,000)
Gain/(Loss)	(100,000)			(100,000)
Square Footage, whole units	64,622			35,761		28,861
Balance Sheet data (as of the period ended):
Land	165,759,000		163,020,000					4,766,000	7,099,000
Buildings, improvements and lease intangibles	2,326,632,000		2,310,404,000					27,117,000	35,424,000
Personal property	18,116,000		17,919,000					425,000	429,000
Real estate properties	2,615,248,000		2,571,605,000					32,308,000	42,952,000
Less accumulated depreciation	(486,572,000)		(484,641,000)					(15,883,000)	(19,447,000)
Assets held for sale, net	2,128,676,000		2,086,964,000					16,425,000	23,505,000
Other assets, net (including receivables)								60,000	410,000
Assets of discontinued operations, net								60,000	410,000
Assets held for sale and discontinued operations, net	16,485,000		23,915,000					16,485,000	23,915,000
Accounts payable and accrued liabilities	66,768,000		62,652,000					107,000	229,000
Other liabilities	50,741,000		43,639,000					108,000	194,000
Liabilities of discontinued operations	$ 215,000		$ 423,000					$ 215,000	$ 423,000