ACQUISITION AND DISPOSITION OF REAL ESTATE AND PROPERTIES HELD FOR SALE - Acquisition (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($) ft² residential_unit	Jun. 30, 2015 USD ($) ft² residential_unit	May. 31, 2015	Apr. 30, 2015	Mar. 31, 2015	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	337,427	337,427
Core Portfolio
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	265,500	265,500
Purchase Price		$ 184,643
Assumption of Debt		9,765
Acquisition costs		$ 700
Fund II
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	0	0
Purchase Price		$ 100,800
Assumption of Debt		$ 81,000
Fund IV
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	59,669	59,669
Purchase Price		$ 84,036
Assumption of Debt		0
Acquisition costs		$ 2,000
City Center | San Fransisco, CA | Core Portfolio
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	205,000	205,000
Percentage of interests acquired	100.00%	100.00%
Purchase Price		$ 155,000
Assumption of Debt		$ 0
163 Highland Avenue | Needham, MA | Core Portfolio
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	40,500	40,500
Percentage of interests acquired	100.00%	100.00%
Purchase Price		$ 24,000
Assumption of Debt		$ 9,765
Route 202 Shopping Center | Wilmington, DE | Core Portfolio
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	20,000	20,000
Percentage of interests acquired	100.00%	100.00%		77.78%
Purchase Price		$ 5,643
Assumption of Debt		$ 0
City Point - Tower I | Brooklyn, NY | Fund II
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	0	0
Percentage of interests acquired	95.00%	95.00%	43.00%
Purchase Price		$ 100,800
Assumption of Debt		$ 81,000
Ownership interest in investment, percentage					52.00%
Total investment, percentage	95.00%	95.00%
Number of residential units | residential_unit	250	250
1035 Third Avenue | New York, NY | Fund IV
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	53,294	53,294
Percentage of interests acquired	100.00%	100.00%
Purchase Price		$ 51,036
Assumption of Debt		$ 0
801 Madison Avenue | New York, NY | Fund IV
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	6,375	6,375
Percentage of interests acquired	100.00%	100.00%
Purchase Price		$ 33,000
Assumption of Debt		$ 0
Acquisitions in 2015
Business Acquisition [Line Items]
Gross leasable area (in square feet) | ft²	325,169	325,169
Purchase Price		$ 369,479
Assumption of Debt		90,765
Purchase Price Allocations
Land	$ 65,759	65,759
Buildings and improvements	204,532	204,532
Above-below market debt assumed (included in Mortgage and other notes payable)	9,765	9,765
Total consideration	260,526	260,526
Acquisitions In 2014
Purchase Price Allocations
Land	60,571	60,571				$ 43,820
Buildings and improvements	105,377	105,377				126,955
Acquisition-related intangible assets (in Acquired lease intangibles, net)	9,836	9,836				0
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	5,009	5,009				0
Total consideration	$ 170,775	170,775				$ 170,775
Adjustments
Land		16,751
Buildings and improvements		(21,578)
Acquisition-related intangible assets (in Acquired lease intangibles, net)		9,836
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)		(5,009)
Total consideration		$ 0
Corporate Joint Venture
Business Acquisition [Line Items]
Percentage of interests acquired	10.00%	10.00%
Purchase Price	$ 4,200
Individually Less Than 3 Percent, Variable Interest Rate
Business Acquisition [Line Items]
Loans converted	1,900	$ 1,900
Individually Less Than 3 Percent, Variable Interest Rate | Corporate Joint Venture
Business Acquisition [Line Items]
Loans converted	$ 1,900	$ 1,900