Real Estate - Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Purchase Price	$ 756,842	$ 451,220
Debt Assumed	120,672	91,885
Acquisition related costs	8,200	4,800
Consideration:
Debt Assumed	(120,672)	(91,885)
Purchase Price	756,842	451,220
Core Portfolio [Member]
Business Acquisition [Line Items]
Purchase Price	519,592	204,243
Debt Assumed	119,209	9,765
Acquisition related costs	5,500	1,300	$ 4,800
Consideration:
Debt Assumed	(119,209)	(9,765)
Purchase Price	519,592	204,243
Fund IV [Member]
Business Acquisition [Line Items]
Purchase Price	237,250	146,177
Debt Assumed	1,463	1,120
Acquisition related costs	2,700	3,500	2,700
Consideration:
Debt Assumed	(1,463)	(1,120)
Purchase Price	$ 237,250	$ 146,177
Fund III [Member]
Business Acquisition [Line Items]
Acquisition related costs			$ 200
991 Madison Avenue [Member] | New York, New York [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 76,628
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	2,902
Consideration:
Debt 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
Debt Assumed	14,556
Consideration:
Debt 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
Debt Assumed	25,650
Consideration:
Debt 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
Debt Assumed	13,409
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	2,692
Consideration:
Debt 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
Debt Assumed	60,000
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	1,463
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt 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
Debt Assumed	0
Consideration:
Debt Assumed	0
Purchase Price	103,500
City Center [Member] | San Francisco, California [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 155,000
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 155,000
163 Highland Avenue [Member] | Needham, MA [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 24,000
Debt Assumed		9,765
Consideration:
Debt Assumed		(9,765)
Purchase Price		$ 24,000
Route 202 Shopping Center [Member] | Wilmington, DE [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 5,643
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 5,643
Roosevelt Galleria [Member] | Chicago, Illinois [Member] | Core Portfolio [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 19,600
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 19,600
City Point Tower I [Member] | Brooklyn, New York [Member] | Fund II [Member]
Business Acquisition [Line Items]
Percent Acquired		95.00%
Purchase Price		$ 100,800
Debt Assumed		81,000
Consideration:
Debt Assumed		(81,000)
Purchase Price		$ 100,800
1035 Third Avenue [Member] | New York, New York [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 51,036
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 51,036
801 Madison Avenue [Member] | New York, New York [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 33,000
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 33,000
650 Bald Hill Road [Member] | Warwick, RI [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		90.00%
Purchase Price		$ 9,216
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 9,216
2208-2216 Filmore Street [Member] | San Francisco, California [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		90.00%
Purchase Price		$ 8,625
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 8,625
146 Geary Street [Member] | San Francisco, California [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		100.00%
Purchase Price		$ 38,000
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		$ 38,000
2207 Fillmore Street [Member] | San Francisco, California [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		90.00%
Purchase Price		$ 2,800
Debt Assumed		1,120
Consideration:
Debt Assumed		(1,120)
Purchase Price		$ 2,800
1861 Union Street [Member] | San Francisco, California [Member] | Fund IV [Member]
Business Acquisition [Line Items]
Percent Acquired		90.00%
Purchase Price		$ 3,500
Debt Assumed		0
Consideration:
Debt Assumed		0
Purchase Price		3,500
Acquisitions in 2016 [Member]
Business Acquisition [Line Items]
Purchase Price	558,755
Debt Assumed	119,209
Purchase Price Allocation
Land	225,729
Buildings and improvements	458,525
Other assets	3,481
Acquisition-related intangible assets (in Acquired lease intangibles, net)	63,606
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)	(72,985)
Above and below market debt assumed (included in Mortgages and other notes payable, net)	(119,601)
Net assets acquired	558,755
Consideration:
Cash	677,964
Debt Assumed	(119,209)
Purchase Price	$ 558,755
Acquisitions In 2015 [Member]
Business Acquisition [Line Items]
Purchase Price		331,931
Debt Assumed		10,885
Purchase Price Allocation
Land		88,068
Buildings and improvements		244,903
Other assets		0
Acquisition-related intangible assets (in Acquired lease intangibles, net)		22,660
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)		(12,094)
Above and below market debt assumed (included in Mortgages and other notes payable, net)		(11,606)
Net assets acquired		331,931
Consideration:
Cash		342,816
Debt Assumed		(10,885)
Purchase Price		331,931
Acquisitions In 2015 [Member] | Previously Reported [Member]
Purchase Price Allocation
Land		83,890
Buildings and improvements		258,926
Other assets		0
Acquisition-related intangible assets (in Acquired lease intangibles, net)		0
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)		0
Above and below market debt assumed (included in Mortgages and other notes payable, net)		(10,885)
Net assets acquired		331,931
Acquisitions In 2015 [Member] | Adjustment [Member]
Purchase Price Allocation
Land		4,178
Buildings and improvements		(14,023)
Other assets		0
Acquisition-related intangible assets (in Acquired lease intangibles, net)		22,660
Acquisition-related intangible liabilities (in Acquired lease intangibles, net)		(12,094)
Above and below market debt assumed (included in Mortgages and other notes payable, net)		(721)
Net assets acquired		$ 0