Real Estate - Acquisitions (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Business Acquisition [Line Items]
Purchase Price	$ 35,350		$ 558,755
Liabilities assumed	663		0
Acquisition related costs	300	$ 300	300
Purchase Price Allocation
Land	7,149		225,729
Buildings and improvements	22,201		458,525
Other assets	0		3,481
Acquisition-related intangible assets (in Acquired lease intangibles, net)	7,444		63,606
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(1,444)		(72,985)
Above and below market debt assumed (included in Mortgages and other notes payable, net)	0		(119,601)
Net assets acquired	35,350		558,755
Consideration:
Cash	34,687		677,964
Debt assumed	0		(119,209)
Liabilities assumed	663		0
Purchase Price	35,350		$ 558,755
Core Portfolio [Member]
Business Acquisition [Line Items]
Acquisition related costs	200	200
Fund III and IV [Member]
Business Acquisition [Line Items]
Acquisition related costs	100	$ 100
Acquisitions in 2016 [Member]
Business Acquisition [Line Items]
Purchase Price	756,842
Liabilities assumed	120,672
Consideration:
Liabilities assumed	120,672
Purchase Price	756,842
Acquisitions in 2016 [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Purchase Price	237,250
Liabilities assumed	1,463
Consideration:
Liabilities assumed	1,463
Purchase Price	237,250
Acquisitions in 2016 [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Purchase Price	519,592
Liabilities assumed	119,209
Consideration:
Liabilities assumed	119,209
Purchase Price	$ 519,592
Lincoln Park Centre [Member] | Fairview Heights, IL [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 35,350
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	35,350
Business Acquisition 2017 [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Purchase Price	35,350
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 35,350
991 Madison Avenue [Member] | New York, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 76,628
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 76,628
165 Newbury Street [Member] | Boston, Massachusetts [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 6,250
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 6,250
Concord and Milwaukee [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 6,000
Liabilities assumed	2,902
Consideration:
Liabilities assumed	2,902
Purchase Price	$ 6,000
151 North State Street [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 30,500
Liabilities assumed	14,556
Consideration:
Liabilities assumed	14,556
Purchase Price	$ 30,500
State and Washington [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 70,250
Liabilities assumed	25,650
Consideration:
Liabilities assumed	25,650
Purchase Price	$ 70,250
North & Kingsbury [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 34,000
Liabilities assumed	13,409
Consideration:
Liabilities assumed	13,409
Purchase Price	$ 34,000
Sullivan Center [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 146,939
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 146,939
California and Armitage [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 9,250
Liabilities assumed	2,692
Consideration:
Liabilities assumed	2,692
Purchase Price	$ 9,250
555 9th Avenue [Member] | San Francisco, California [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 139,775
Liabilities assumed	60,000
Consideration:
Liabilities assumed	60,000
Purchase Price	$ 139,775
Restaurant at Fort Point [Member] | Boston, Massachusetts [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 11,500
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 11,500
1964 Union Street [Member] | San Francisco, California [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	90.00%
Purchase Price	$ 2,250
Liabilities assumed	1,463
Consideration:
Liabilities assumed	1,463
Purchase Price	$ 2,250
Wake Forest Crossing [Member] | Wake Forest, NC [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 36,600
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 36,600
Airport Mall [Member] | Bangor, ME [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 10,250
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 10,250
Colonie Plaza [Member] | Albany, NY [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 15,000
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 15,000
Dauphin Plaza [Member] | Harrisburg, PA [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 16,000
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 16,000
JFK Plaza [Member] | Waterville, ME [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 6,500
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 6,500
Mayfair Shopping Center [Member] | Philadelphia, PA [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 16,600
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 16,600
Shaw's Plaza [Member] | Waterville, ME [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 13,800
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 13,800
Wells Plaza [Member] | Wells, ME [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 5,250
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 5,250
717 N Michigan [Member] | Chicago, Illinois [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 103,500
Liabilities assumed	0
Consideration:
Liabilities assumed	0
Purchase Price	$ 103,500