Investments in Real Estate Partnerships - Schedule of Acquisitions by Real Estate Partnerships (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 116,239,000	$ 191,384,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	9,700,000	27,000,000
Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	346,862,000	15,075,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 100,233,000	$ 0
Unconsolidated Properties [Member] | Ballard Blocks I [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 02, 2018
Business Acquisition, Name of Acquired Entity	Ballard Blocks I
Business Acquisition Location, City and State	Seattle, WA
Business Acquisition, Description of Acquired Entity	Operating
Business Acquisition, Co-investment Partner	Other
Equity Method Investment, Ownership Percentage	49.90%
Payments to Acquire Businesses, Gross	$ 54,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0
Unconsolidated Properties [Member] | Ballard Blocks II [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 02, 2018
Business Acquisition, Name of Acquired Entity	Ballard Blocks II
Business Acquisition Location, City and State	Seattle, WA
Business Acquisition, Description of Acquired Entity	Development
Business Acquisition, Co-investment Partner	Other
Equity Method Investment, Ownership Percentage	49.90%
Payments to Acquire Businesses, Gross	$ 4,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0
Unconsolidated Properties [Member] | Metuchen [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 05, 2018
Business Acquisition, Name of Acquired Entity	The District at Metuchen
Business Acquisition Location, City and State	Metuchen, NJ
Business Acquisition, Description of Acquired Entity	Operating
Business Acquisition, Co-investment Partner	Columbia II
Equity Method Investment, Ownership Percentage	20.00%
Payments to Acquire Businesses, Gross	$ 33,830,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0
Unconsolidated Properties [Member] | Crossroad Commons II [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	May 18, 2018
Business Acquisition, Name of Acquired Entity	Crossroads Commons II
Business Acquisition Location, City and State	Boulder, CO
Business Acquisition, Description of Acquired Entity	Operating
Business Acquisition, Co-investment Partner	Columbia I
Equity Method Investment, Ownership Percentage	20.00%
Payments to Acquire Businesses, Gross	$ 10,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0
Unconsolidated Properties [Member] | Ridgewood [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Sep. 07, 2018
Business Acquisition, Name of Acquired Entity	Ridgewood Shopping Center
Business Acquisition Location, City and State	Raleigh, NC
Business Acquisition, Description of Acquired Entity	Operating
Business Acquisition, Co-investment Partner	Columbia II
Equity Method Investment, Ownership Percentage	20.00%
Payments to Acquire Businesses, Gross	$ 45,800,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 10,233,000
Unconsolidated Properties [Member] | Shoppes at Bartram Park [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Dec. 17, 2018
Business Acquisition, Name of Acquired Entity	Shoppes at Bartram Park
Business Acquisition Location, City and State	Jacksonville, FL
Business Acquisition, Description of Acquired Entity	Operating (1)
Business Acquisition, Co-investment Partner	Other
Equity Method Investment, Ownership Percentage	50.00%
Payments to Acquire Businesses, Gross	$ 984,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0
Unconsolidated Properties [Member] | Town and Country [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Dec. 14, 2018
Business Acquisition, Name of Acquired Entity	Town and Country Center
Business Acquisition Location, City and State	Los Angeles, CA
Business Acquisition, Description of Acquired Entity	Operating
Business Acquisition, Co-investment Partner	Other
Equity Method Investment, Ownership Percentage	9.38%
Payments to Acquire Businesses, Gross	$ 197,248,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	90,000,000
Unconsolidated Properties [Member] | Midtown East [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Oct. 11, 2017
Business Acquisition, Name of Acquired Entity		Midtown East
Business Acquisition Location, City and State		Raleigh, NC
Business Acquisition, Description of Acquired Entity		Development
Business Acquisition, Co-investment Partner		Other
Equity Method Investment, Ownership Percentage		50.00%
Payments to Acquire Businesses, Gross		$ 15,075,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		0
Off-Market Favorable Lease [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	8,371,000	8,929,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	13,889,000	0
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Ballard Blocks I [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	3,668,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Ballard Blocks II [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Metuchen [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	3,147,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Crossroad Commons II [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	447,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Ridgewood [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	3,372,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Shoppes at Bartram Park [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Town and Country [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	3,255,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member] | Midtown East [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		0
Off-Market Lease, Unfavorable [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	7,422,000	17,562,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	12,952,000	0
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Ballard Blocks I [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	2,350,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Ballard Blocks II [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Metuchen [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	1,905,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Crossroad Commons II [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	769,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Ridgewood [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	2,278,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Shoppes at Bartram Park [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Town and Country [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	$ 5,650,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Midtown East [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		$ 0