ACQUISITION AND DISPOSITION OF REAL ESTATE AND DISCONTINUED OPERATIONS Acquisitions (Details) (USD $)	9 Months Ended	1 Months Ended														9 Months Ended		1 Months Ended		1 Months Ended
	Sep. 30, 2012 Core Portfolio	Jul. 31, 2012 83 Spring Street New York New York Core Portfolio sqft	Jun. 30, 2012 Shopping Center Washington, D.C. Core Portfolio sqft	May 31, 2012 28 Jericho Turnpike Westbury, New York Core Portfolio sqft	Apr. 30, 2012 930 Rush Street Chicago Illinois Core Portfolio sqft	Mar. 31, 2012 Four Property Portfolio Chicago Illinois Core Portfolio	Feb. 29, 2012 River Street Cambridge Massachusetts Core Portfolio sqft	Jan. 31, 2012 1520 North Milwaukee Avenue Chicago Illinois Core Portfolio sqft	Sep. 30, 2012 Acquisitions in 2012 Purchase Price Allocation As Originally Reported	Sep. 30, 2012 Acquisitions in 2012 Adjustments	Sep. 30, 2012 Acquisitions in 2012 Finalized Purchase Price Allocation	Sep. 30, 2012 Acquisitions in 2012 Preliminary Purchase Price Allocation		Sep. 30, 2012 Acquisitions in 2011 Finalized Purchase Price Allocation	Dec. 31, 2011 Acquisitions in 2011 Preliminary Purchase Price Allocation	Sep. 30, 2012 Fund III	Sep. 30, 2012 Fund III Opportunity Funds	Aug. 31, 2012 Fund III 3104 M Street Washington, D.C. Opportunity Funds sqft	Aug. 31, 2012 Fund III Undeveloped land Mohegan Lake, New York Opportunity Funds	Aug. 31, 2012 Fund III Undeveloped land Farmingdale, New York Opportunity Funds acre	Aug. 31, 2012 Fund III Arundel Plaza Glen Burnie, Maryland Opportunity Funds sqft	Apr. 30, 2012 Fund III Lincoln Park Centre Chicago Illinois Opportunity Funds sqft	Feb. 29, 2012 Fund III 640 Broadway New York New York Opportunity Funds sqft
Business Acquisition [Line Items]
Business Acquisition, Cost of Acquired Entity, Purchase Price		$ 11,500,000	$ 21,700,000	$ 27,300,000	$ 20,700,000	$ 18,800,000	$ 18,900,000	$ 3,800,000										$ 3,000,000	$ 11,000,000		$ 17,600,000	$ 31,500,000	$ 16,300,000
Business Acquisition, Debt Assumption			16,500,000			16,000,000	7,000,000													10,000,000	9,300,000	19,800,000
Acrage of Real Estate Property																				19.2
Square Footage of Real Estate Property (in square feet)		4,800	57,000	96,000	2,900		53,300	3,100										4,900			265,000	62,700	45,700
Business Combination, Acquisition Related Costs	1,200,000																2,200,000
Business Acquisition, Percentage of Voting Interests Acquired																					90.00%
Ownership Interest By General Partner or Managing Partner																19.90%							50.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net
Land									50,870,000	4,951,000	55,821,000	27,084,000	[1]	12,150,000	5,438,000
Buildings and Improvements									67,303,000	(7,432,000)	59,871,000	85,592,000	[1]	11,009,000	18,563,000
Acquisition-related intangible assets (in Acquired lease intangibles, net)									2,482,000	3,463,000	5,945,000	0	[1]	1,027,000	0
Acquisition-related intangible liabilities (in Acquired lease and other intangibles, net)									(4,387,000)	(1,056,000)	(5,443,000)	0	[1]	(185,000)	0
Above-below market debt assumed (included in Mortgage notes payable)									935,000	74,000	1,009,000	0	[1]
Total consideration									$ 117,203,000	$ 0	$ 117,203,000	$ 112,676,000	[1]	$ 24,001,000	$ 24,001,000

[1] Represents preliminary allocation for 2012 acquisitions where purchase price allocations have not been finalized