MORTGAGES NOTES PAYABLE (Details) (USD $)			1 Months Ended		9 Months Ended		1 Months Ended		1 Months Ended			1 Months Ended							1 Months Ended			1 Months Ended								1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Aug. 31, 2012 Mortgages	Mar. 31, 2012 Chicago Illinois Four Property Portfolio properties	Sep. 30, 2012 Secured Credit Facilities - Variable-Rate Fund III revolving subscription line of credit Line of Credit	Sep. 30, 2012 Secured Credit Facilities - Variable-Rate Fund II Line of Credit	Sep. 30, 2012 6.40% Mortgage Note Mortgages	Sep. 30, 2012 4.20% Mortgage Note Mortgages	Jun. 30, 2012 LIBOR Plus 2.95% Mortgage Note Mortgages	Jun. 30, 2012 LIBOR Plus 2.95% Mortgage Note Mortgages LIBOR	Sep. 30, 2012 LIBOR Plus 2.95% Mortgage Note Mortgages LIBOR	Jun. 30, 2012 6.35% Mortgage Note Washington, D.C. Shopping Center Mortgages	Jun. 30, 2012 Disposal Property Loan, Mortgage Note Mortgages	May 31, 2012 3.68% Mortgage Note Mortgages	Apr. 30, 2012 LIBOR Plus 2.25% Construction Loan Construction Loans extension_options	Apr. 30, 2012 LIBOR Plus 2.25% Construction Loan Construction Loans LIBOR	Apr. 30, 2012 Mortgage Notes Maturing April 2012 Mortgages	Sep. 30, 2012 Mortgage Notes Maturing April 2012 Mortgages	Apr. 30, 2012 Mortgage Notes Maturing April 2012 Chicago Illinois Lincoln Park Centre Fund III revolving subscription line of credit Mortgages	Sep. 30, 2012 LIBOR Plus 1.9% Mortgage Note Mortgages	Apr. 30, 2012 LIBOR Plus 1.9% Mortgage Note Mortgages	Sep. 30, 2012 LIBOR Plus 1.9% Mortgage Note Mortgages LIBOR	Apr. 30, 2012 LIBOR Plus 1.9% Mortgage Note Mortgages LIBOR	Sep. 30, 2012 LIBOR Plus 3.0% Mortgage Note Mortgages extension_options	Sep. 30, 2012 LIBOR Plus 3.0% Mortgage Note Mortgages LIBOR	Aug. 31, 2012 LIBOR Plus 2.25% Mortgage Note Mortgages extension_options	Aug. 31, 2012 LIBOR Plus 2.25% Mortgage Note Mortgages LIBOR	Aug. 31, 2012 1% Mortgage Note Mortgages	Aug. 31, 2012 LIBOR Plus Three Point Three Percent Construction Line of Credit [Member] Construction Loans	Aug. 31, 2012 LIBOR Plus Three Point Three Percent Construction Line of Credit [Member] Construction Loans LIBOR	Jul. 31, 2012 LIBOR Plus Three Point Three-Five Percent Mortgage Note [Member] Mortgages	Jul. 31, 2012 LIBOR Plus Three Point Three-Five Percent Mortgage Note [Member] Mortgages LIBOR	Mar. 31, 2012 5.62% Mortage Note Chicago Illinois Four Property Portfolio Mortgages	Mar. 31, 2012 5.55% Mortage Note Chicago Illinois Four Property Portfolio Mortgages	Feb. 29, 2012 6.26% Mortage Note Cambridge Massachusetts Single Tenant Property Mortgages	Feb. 29, 2012 Mortgage Notes Payable - Variable-Rate Mortgages LIBOR
Debt Instrument [Line Items]
Line of credit facility, repayments					$ 53,100,000	$ 7,600,000
Line of credit facility, amount outstanding					82,900,000	32,400,000
Long-term Debt, Refinanced Amount							7,800,000								56,500,000									83,300,000							11,500,000
Mortgages payable, face amount							8,000,000		22,800,000					4,300,000	69,600,000					6,400,000	15,500,000					21,000,000		20,000,000	50,000,000		10,600,000
Long-term Debt, Intercompany Elimination																												14,800,000
Mortgages payable	871,243,000	767,150,000																73,900,000										5,200,000
Business Acquisition, Debt Assumption												16,500,000							19,800,000														14,500,000	1,500,000	7,000,000
Interest rate							6.40%	4.20%				6.35%		3.68%					5.85%									1.00%			7.34%		5.62%	5.55%	6.26%
Repayments of Long-term Debt			12,500,000										8,300,000
Debt Instrument, Increase, Additional Borrowings																	$ 23,600,000
Number of Properties Collateralized (in properties)				4
Number of loan extensions (in extension options)															2									2		2
Length of loan extension option									1 year					5 years	1 year									1 year		1 year									5 years
LIBOR										LIBOR						LIBOR						LIBOR	LIBOR		LIBOR		LIBOR			LIBOR		LIBOR				LIBOR
Basis Spread											2.95%					2.25%						1.90%	1.90%		3.00%		2.25%			3.30%		3.35%				2.00%