Real Estate - Acquisitions and Conversions (Details) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Purchase Price	$ 46,596,000		$ 254,532,000
Debt assumed	0		0
Capitalized acquisition costs	100,000
Acquisition costs expensed		$ 600,000
Purchase Price Allocation
Land	8,924,000		48,138,000
Buildings and improvements	34,237,000		173,576,000
Other assets	0		84,000
Acquisition-related intangible assets (Note 6)	6,486,000		44,269,000
Acquisition-related intangible liabilities (Note 6)	(3,051,000)		(11,535,000)
Net assets acquired	46,596,000		254,532,000
Consideration
Cash	46,171,000		200,429,000
Conversion of note receivable	0		41,010,000
Liabilities assumed	425,000		3,363,000
Existing interest in previously unconsolidated investment	0		4,159,000
Change in control of previously unconsolidated investment	0		5,571,000
Total Consideration	46,596,000		254,532,000
Core Portfolio
Business Acquisition [Line Items]
Purchase Price	$ 1,337,000		$ 42,800,000
Acquisition costs expensed		200,000
Core Portfolio | Bedford Green Land Parcel
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 1,337,000
Core Portfolio | Market Square Shopping Center | Wilmington, DE
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 42,800,000
Funds Segment
Business Acquisition [Line Items]
Capitalized acquisition costs	100,000
Acquisition costs expensed		$ 400,000
Fund IV
Business Acquisition [Line Items]
Purchase Price			$ 44,492,000
Fund IV | Lincoln Place - Fairview Heights, IL | Fairview Heights, IL
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 35,350,000
Fund IV | Shaw's Plaza Waterville, ME | Windham, ME
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 9,142,000
Fund V
Business Acquisition [Line Items]
Purchase Price	$ 45,259,000		$ 167,240,000
Fund V | Trussville Promenade - Trussville, AL | Trussville, AL
Business Acquisition [Line Items]
Percent Acquired	100.00%
Purchase Price	$ 45,259,000
Fund V | Plaza Santa Fe - Santa Fe, NM | Santa Fe, NM
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 35,220,000
Fund V | Hickory Ridge - Hickory, NC | Hickory, NC
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 44,020,000
Fund V | New Towne Plaza - Canton, MI | Canton, MI
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 26,000,000
Fund V | Fairlane Green | Allen Park, MI
Business Acquisition [Line Items]
Percent Acquired			100.00%
Purchase Price			$ 62,000,000