ACQUISITION AND DISPOSITION OF PROPERTIES, DISCONTINUED OPERATIONS AND PROPERTIES HELD FOR SALE Acquisitions (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended		1 Months Ended
	Dec. 31, 2014 sqft		Dec. 31, 2014		Dec. 31, 2013
Business Acquisition [Line Items]
GLA (in square feet)	510,219		510,219
Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	368,551		368,551
Purchase Price	$ 473,200,000
Debt Assumption	84,800,000
Acquisition related costs	4,800,000
Fund III [Member]
Business Acquisition [Line Items]
Acquisition related costs	200,000
Fund IV [Member]
Business Acquisition [Line Items]
GLA (in square feet)	472,551		472,551
Purchase Price	106,600,000
Acquisition related costs	2,700,000
11 East Walton [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	6,738		6,738
Percent Owned	100.00%		100.00%
Purchase Price	44,000,000
Debt Assumption	0
61 Main Street [Member] | West Port, Connecticut [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	3,400		3,400
Percent Owned	100.00%		100.00%
Purchase Price	7,300,000
Debt Assumption	0
865 W. North Avenue [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	16,000		16,000
Percent Owned	100.00%		100.00%
Purchase Price	14,800,000
Debt Assumption	0
252 - 254 Greenwich Avenue [Member] | Greenwich, Connecticut [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	9,172		9,172
Percent Owned	100.00%		100.00%
Purchase Price	24,500,000
Debt Assumption	0
152 - 154 Spring Street [Member] | New York, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	2,936		2,936
Percent Owned	90.00%		90.00%
Purchase Price	38,000,000
Debt Assumption	0
2520 Flatbush Avenue [Member] | Brooklyn, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	29,114		29,114
Percent Owned	100.00%		100.00%
Purchase Price	17,100,000
Debt Assumption	0
Bedford Green [Member] | Bedford, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	90,472		90,472
Percent Owned	100.00%		100.00%
Purchase Price	46,800,000
Debt Assumption	29,800,000
131 - 135 Prince Street [Member] | Common OP Units [Member]
Business Acquisition [Line Items]
Issuance of OP units in exchange for ownership, number of shares	1.4
131 - 135 Prince Street [Member] | New York, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	3,200	[1]	3,200	[1]
Percent Owned	100.00%	[1]	100.00%	[1]
Purchase Price	51,400,000	[1]
Debt Assumption	0	[1]
Shops At Grand Avenue [Member] | Queens, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	99,975		99,975
Percent Owned	100.00%		100.00%
Purchase Price	56,000,000
Debt Assumption	0
201 Needham Street [Member] | Newton, Massachusetts [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	20,409		20,409
Percent Owned	100.00%		100.00%
Purchase Price	10,100,000
Debt Assumption	0
840 North Michigan [Member] | Common OP Units [Member]
Business Acquisition [Line Items]
Issuance of OP units in exchange for ownership, number of shares	0.2
840 North Michigan [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
GLA (in square feet)	87,135	[2]	87,135	[2]
Percent Owned	88.00%	[2]	88.00%	[2]
Purchase Price	163,200,000	[2]
Debt Assumption	55,000,000	[2]
Arundel Plaza [Member] | Fund III [Member]
Business Acquisition [Line Items]
Purchase Price			3,100,000
Broughton Street Portfolio [Member] | Savannah, Georgia [Member] | Fund IV [Member]
Business Acquisition [Line Items]
GLA (in square feet)	218,076	[3]	218,076	[3]
Percent Owned	50.00%	[3]	50.00%	[3]
Purchase Price	33,900,000	[3]
Number of properties acquired	24
Number of asset acquisitions	9
Eden Square [Member] | Bear, Delaware [Member] | Fund IV [Member]
Business Acquisition [Line Items]
GLA (in square feet)	235,508	[4]	235,508	[4]
Percent Owned	90.00%	[4]	90.00%	[4]
Purchase Price	25,400,000	[4]
17 E. 71st Street [Member] | New York, New York [Member] | Fund IV [Member]
Business Acquisition [Line Items]
GLA (in square feet)	9,330		9,330
Percent Owned	100.00%		100.00%
Purchase Price	28,000,000
27 E. 61st Street [Member] | New York, New York [Member] | Fund IV [Member]
Business Acquisition [Line Items]
GLA (in square feet)	9,637	[5]	9,637	[5]
Percent Owned	100.00%	[5]	100.00%	[5]
Purchase Price	19,300,000	[5]
Acquisitions in 2013 [Member]
Purchase Price Allocation
Land	80,524,000		80,524,000		65,804,000
Buildings and Improvements	180,605,000		180,605,000		245,925,000
Acquisition-related intangible assets (in Acquired lease intangibles, net)	56,871,000		56,871,000		0
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(6,271,000)		(6,271,000)		0
Total Consideration	311,729,000		311,729,000		311,729,000
Adjustments
Land	14,720,000
Buildings and Improvements	(65,320,000)
Acquisition-related intangible assets (in Acquired lease intangibles, net)	56,871,000
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(6,271,000)
Total Consideration	0
Acquisitions In 2014 [Member]
Purchase Price Allocation
Land	145,833,000		145,833,000
Buildings and Improvements	411,896,000		411,896,000
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(6,434,000)		(6,434,000)
Above-below market debt assumed (included in Mortgages and other notes payable)	(2,100,000)		(2,100,000)
Total Consideration	$ 549,195,000		$ 549,195,000

[1] This acquisition was primarily funded with the issuance of 1.4 million Common OP Units.

[2] As the tenancy in common partner to this investment maintains operating control over this investment, it is accounted for under the equity method. This acquisition was partially funded with the issuance of 0.2 million Common OP Units.

[3] The Broughton Street Portfolio consists of 24 properties. As the joint venture partner to these investments maintains operating control over the investments, these are accounted for under the equity method. Of the 24 properties, nine of them are accounted for as asset acquisitions as they were purchased vacant and require future development.

[4] As the joint venture partner to this investment maintains operating control over this investment, it is accounted for under the equity method.

[5] This property was purchased vacant and requires future development and as such, it is accounted for as an asset acquisition.