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Discontinued Operations and Assets Held for Sale (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended				1 Months Ended
	Sep. 30, 2011 Year property segment	Jun. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 sqfeet Year property segment	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Assets held for sale	Sep. 30, 2011 Towson Portfolio property
Discontinued Operations and Assets Held for Sale
Number of operating properties held for sale	15			15
Revenue from real estate operations	$ 3,236		$ 3,954	$ 9,434	$ 12,344
Property operating expenses	1,057		1,291	3,784	4,413
Depreciation and amortization	313		1,242	3,381	3,528
Impairment loss		14,500		14,523
Expenses from real estate operations	1,370		2,533	21,688	7,941
Operating income from real estate operations	1,866		1,421	(12,254)	4,403
Interest expense	(248)		(452)	(975)	(1,208)
Loss on early extinguishment of debt	(340)			(340)
Gain on sales of real estate	1,299		784	1,449	1,081
Discontinued operations	2,577		1,753	(12,120)	4,276
Properties
Properties, net	3,469,217			3,469,217		3,445,455	70,278
Deferred rent receivable	87,148			87,148		79,160	974
Intangible assets on real estate acquisitions, net	97,954			97,954		113,735	543
Deferred leasing costs, net							972
Assets held for sale, net	$ 72,767			$ 72,767			$ 72,767
Number of properties sold								4