Real Estate - Acquisitions (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Business Acquisition [Line Items]
Purchase Price	$ 149,732		$ 756,842
Debt assumed	0		120,672
Acquisition related costs	900	$ 5,500
Purchase Price Allocation
Land	21,917		225,729
Buildings and improvements	104,729		458,525
Other assets	0		3,481
Acquisition-related intangible assets (in Acquired lease intangibles, net)	31,378		63,606
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(8,292)		(72,985)
Above and below market debt assumed (included in Mortgages and other notes payable, net)	0		(119,601)
Net assets acquired	149,732		558,755
Consideration:
Cash	138,429		439,546
Conversion of note receivable	9,142		0
Debt assumed	0		120,672
Liabilities assumed	2,161		0
Total Consideration	149,732		558,755
Core Portfolio and Opportunity Funds
Business Acquisition [Line Items]
Acquisition related costs	900	5,500
Core Portfolio
Business Acquisition [Line Items]
Purchase Price			519,592
Debt assumed	0		119,209
Acquisition related costs	300	5,100
Consideration:
Debt assumed	0		$ 119,209
Core Portfolio | 991 Madison Avenue - New York, NY | New York, NY
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 76,628
Debt assumed			0
Consideration:
Debt assumed			$ 0
Core Portfolio | 165 Newbury Street - Boston, MA | Boston, MA
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 6,250
Debt assumed			0
Consideration:
Debt assumed			$ 0
Core Portfolio | Concord & Milwaukee - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 6,000
Debt assumed			2,902
Consideration:
Debt assumed			$ 2,902
Core Portfolio | 151 North State Street - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 30,500
Debt assumed			14,556
Consideration:
Debt assumed			$ 14,556
Core Portfolio | State & Washington - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 70,250
Debt assumed			25,650
Consideration:
Debt assumed			$ 25,650
Core Portfolio | North & Kingsbury - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 34,000
Debt assumed			13,409
Consideration:
Debt assumed			$ 13,409
Core Portfolio | Sullivan Center - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 146,939
Debt assumed			0
Consideration:
Debt assumed			$ 0
Core Portfolio | California & Armitage - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 9,250
Debt assumed			2,692
Consideration:
Debt assumed			$ 2,692
Core Portfolio | 555 9th Street - San Francisco, CA | San Francisco, CA
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 139,775
Debt assumed			60,000
Consideration:
Debt assumed			60,000
Funds Segment
Business Acquisition [Line Items]
Acquisition related costs	600	$ 400
Fund IV
Business Acquisition [Line Items]
Purchase Price	44,492		237,250
Debt assumed	0		1,463
Consideration:
Debt assumed	$ 0		$ 1,463
Fund IV | Lincoln Place - Fairview Heights, IL | Fairview Heights, IL
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 35,350
Debt assumed	0
Consideration:
Debt assumed	$ 0
Fund IV | Shaw's Plaza - Waterville, ME | Windham, ME
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 9,142
Debt assumed	0
Consideration:
Debt assumed	0
Fund IV | Shaw's Plaza - Waterville, ME | Waterville, ME
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 13,800
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Restaurants at Fort Point - Boston, MA | Boston, MA
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 11,500
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | 1964 Union Street - San Francisco, CA | San Francisco, CA
Business Acquisition [Line Items]
Percent Acquired			90.00%
Purchase Price			$ 2,250
Debt assumed			1,463
Consideration:
Debt assumed			$ 1,463
Fund IV | Wake Forest Crossing - Wake Forest, NC | Wake Forest, NC
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 36,600
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Airport Mall - Bangor, ME | Bangor, ME
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 10,250
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Colonie Plaza - Albany, NY | Albany, NY
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 15,000
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Dauphin Plaza - Harrisburg, PA | Harrisburg, PA
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 16,000
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | JFK Plaza - Waterville, ME | Waterville, ME
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 6,500
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Mayfair Shopping Center - Philadelphia, PA | Philadelphia, PA
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 16,600
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | Wells Plaza - Wells, ME | Wells, ME
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 5,250
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund IV | 717 N Michigan - Chicago, IL | Chicago, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 103,500
Debt assumed			0
Consideration:
Debt assumed			$ 0
Fund V
Business Acquisition [Line Items]
Purchase Price	105,240
Debt assumed	0
Consideration:
Debt assumed	$ 0
Fund V | Plaza Santa Fe - Santa Fe, NM | Santa Fe, NM
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 35,220
Debt assumed	0
Consideration:
Debt assumed	$ 0
Fund V | Hickory Ridge - Hickory, NC | Hickory, NC
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 44,020
Debt assumed	0
Consideration:
Debt assumed	$ 0
Fund V | New Towne Plaza - Canton, MI | Canton, MI
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 26,000
Debt assumed	0
Consideration:
Debt assumed	$ 0