Investments in Real Estate Partnerships - Schedule of Acquisitions by Real Estate Partnerships (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 191,384,000	$ 387,528,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	27,000,000	0
Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	15,075,000	92,550,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 0	$ 35,076,000
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	ITB Holdings, LLC
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
Unconsolidated Properties [Member] | Applewood Village [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Mar. 24, 2016
Business Acquisition, Name of Acquired Entity		Applewood Village Shops
Business Acquisition Location, City and State		Denver, CO
Business Acquisition, Description of Acquired Entity		Operating (1)
Business Acquisition, Co-investment Partner		GRIR
Equity Method Investment, Ownership Percentage		40.00%
Payments to Acquire Businesses, Gross		$ 200,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		$ 0
Unconsolidated Properties [Member] | Garden City [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Dec. 20, 2016
Business Acquisition, Name of Acquired Entity		Plaza Venezia
Business Acquisition Location, City and State		Orlando, FL
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		$ 92,350,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		35,076,000
Off-Market Favorable Lease [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	8,929,000	27,863,000
Off-Market Favorable Lease [Member] | Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0	6,899,000
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 Favorable Lease [Member] | Unconsolidated Properties [Member] | Applewood Village [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] | Garden City [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		6,899,000
Off-Market Lease, Unfavorable [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	17,562,000	19,141,000
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	0	11,548,000
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
Off-Market Lease, Unfavorable [Member] | Unconsolidated Properties [Member] | Applewood Village [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] | Garden City [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		$ 11,548,000