Mortgage Loan and Other Indebtedness Balances (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended				12 Months Ended
	Dec. 31, 2014		Apr. 30, 2014	Feb. 28, 2014	Dec. 31, 2014		Dec. 31, 2013		Dec. 20, 2011
Debt Instrument [Line Items]
Mortgage and other notes payable	1,130,481				1,130,481		$ 1,039,997
Unamortized premium (discount)	2,943				2,943		1,868
LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description	LIBOR		LIBOR	LIBOR
Variable Rate Debt [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	326,221				326,221		224,041
Mortgage Notes Payable - Variable-Rate [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	422,950				422,950		284,951
Mortgage Notes Payable - Variable-Rate [Member] | Liberty Avenue [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	8,973				8,973		9,090
Effective interest rate	2.92%				2.92%
Mortgage Notes Payable - Variable-Rate [Member] | Liberty Avenue [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.75%
Mortgage Notes Payable - Variable-Rate [Member] | 210 Bowery LLC [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	4,600				4,600		4,600
Effective interest rate	2.12%				2.12%
Mortgage Notes Payable - Variable-Rate [Member] | 210 Bowery LLC [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.95%
Mortgage Notes Payable - Variable-Rate [Member] | 640 Broadway [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	22,564				22,564		22,750
Effective interest rate	3.12%				3.12%
Mortgage Notes Payable - Variable-Rate [Member] | 640 Broadway [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.95%
Mortgage Notes Payable - Variable-Rate [Member] | Heritage Shops, LIBOR Plus 2.25% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		20,871
Effective interest rate	2.42%				2.42%
Mortgage Notes Payable - Variable-Rate [Member] | Heritage Shops, LIBOR Plus 2.25% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.25%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 4.00% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	20,650				20,650		20,650
Effective interest rate	4.17%				4.17%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 4.00% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					4.00%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 1.70% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	20,000				20,000		20,000
Effective interest rate	1.87%				1.87%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 1.70% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.70%
Mortgage Notes Payable - Variable-Rate [Member] | Cortlandt Towne Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	83,936				83,936		84,745
Effective interest rate	1.82%				1.82%
Mortgage Notes Payable - Variable-Rate [Member] | Cortlandt Towne Center [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.65%
Mortgage Notes Payable - Variable-Rate [Member] | New Hyde Park Shopping Center, LIBOR Plus 1.85% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		6,294
Effective interest rate	2.40%				2.40%
Mortgage Notes Payable - Variable-Rate [Member] | New Hyde Park Shopping Center, LIBOR Plus 1.85% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.85%
Mortgage Notes Payable - Variable-Rate [Member] | Norstrand Avenue [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	12,046				12,046		12,567
Effective interest rate	2.82%				2.82%
Mortgage Notes Payable - Variable-Rate [Member] | Norstrand Avenue [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.65%
Mortgage Notes Payable - Variable-Rate [Member] | Heritage Shops, LIBOR Plus 1.55% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	24,500				24,500		0
Effective interest rate	1.72%				1.72%
Mortgage Notes Payable - Variable-Rate [Member] | Heritage Shops, LIBOR Plus 1.55% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.55%
Mortgage Notes Payable - Variable-Rate [Member] | Lincoln Park Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	28,000				28,000		23,000
Effective interest rate	1.62%				1.62%
Mortgage Notes Payable - Variable-Rate [Member] | Lincoln Park Center [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.45%
Mortgage Notes Payable - Variable-Rate [Member] | 654 Broadway [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	9,000				9,000		0
Effective interest rate	2.05%				2.05%
Mortgage Notes Payable - Variable-Rate [Member] | 654 Broadway [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.88%
Mortgage Notes Payable - Variable-Rate [Member] | New Hyde Park Shopping Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	11,720				11,720		0
Effective interest rate	2.02%				2.02%
Mortgage Notes Payable - Variable-Rate [Member] | New Hyde Park Shopping Center [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.85%
Mortgage Notes Payable - Variable-Rate [Member] | 938 West North Avenue [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	12,500				12,500		0
Effective interest rate	2.52%				2.52%
Mortgage Notes Payable - Variable-Rate [Member] | 938 West North Avenue [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.35%
Mortgage Notes Payable - Variable-Rate [Member] | 1151 Third Avenue [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	12,481				12,481		0
Effective interest rate	1.92%				1.92%
Mortgage Notes Payable - Variable-Rate [Member] | 1151 Third Avenue [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.75%
Mortgage Notes Payable - Variable-Rate [Member] | 161st Street [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	29,500				29,500		29,500
Effective interest rate	2.67%				2.67%
Mortgage Notes Payable - Variable-Rate [Member] | 161st Street [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.50%
Mortgage Notes Payable - Variable-Rate [Member] | 664 N. Michigan [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	44,369				44,369		0
Effective interest rate	1.82%				1.82%
Mortgage Notes Payable - Variable-Rate [Member] | 664 N. Michigan [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.65%
Mortgage Notes Payable - Variable-Rate [Member] | Paramus Plaza [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	12,600				12,600		0
Effective interest rate	1.87%				1.87%
Mortgage Notes Payable - Variable-Rate [Member] | Paramus Plaza [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.70%
Mortgage Notes Payable - Variable-Rate [Member] | Lake Montclair [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	15,284				15,284		0
Effective interest rate	2.32%				2.32%
Mortgage Notes Payable - Variable-Rate [Member] | Lake Montclair [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					2.15%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 1.39% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	20,000				20,000		0
Effective interest rate	1.56%				1.56%
Mortgage Notes Payable - Variable-Rate [Member] | City Point, LIBOR Plus 1.39% [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.39%
Mortgage Notes Payable - Variable-Rate [Member] | 3104 M Street [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	103				103		0
Effective interest rate	3.75%				3.75%
Mortgage Notes Payable - Variable-Rate [Member] | 3104 M Street [Member] | Prime [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					Prime
Borrowings, interest rate basis spread					0.05%
Mortgage Notes Payable - Variable-Rate [Member] | 4401 White Plains [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	6,141				6,141		6,263
Effective interest rate	2.07%				2.07%
Mortgage Notes Payable - Variable-Rate [Member] | 4401 White Plains [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.90%
Mortgage Notes Payable - Variable-Rate [Member] | 28 Jericho Turnpike [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	15,747				15,747		16,164
Effective interest rate	2.07%				2.07%
Mortgage Notes Payable - Variable-Rate [Member] | 28 Jericho Turnpike [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.90%
Mortgage Notes Payable - Variable-Rate [Member] | 60 Orange Street [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	8,236				8,236		8,457
Effective interest rate	1.92%				1.92%
Mortgage Notes Payable - Variable-Rate [Member] | 60 Orange Street [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.75%
Unsecured Debt [Member] | Fund IV [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	77,100	[1]			77,100	[1]	68,750	[1]
Effective interest rate	1.82%				1.82%
Unsecured Debt [Member] | LIBOR [Member] | Fund IV [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.65%
Unsecured Debt [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		0
Effective interest rate	1.57%				1.57%
Unsecured Debt [Member] | Line of Credit [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.40%
Unsecured Debt [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	50,000				50,000		50,000
Effective interest rate	1.47%				1.47%
Unsecured Debt [Member] | Term Loan [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Borrowings, interest rate description					LIBOR
Borrowings, interest rate basis spread					1.30%
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	223,829	[2]			223,829	[2]	179,660	[2]
Effective interest rate	2.15%	[2]			2.15%	[2]
Mortgage Notes Payable - Fixed Rate [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	801,317				801,317		814,088
Mortgage Notes Payable - Fixed Rate [Member] | Clark Diversey [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		4,192
Effective interest rate	6.35%				6.35%
Mortgage Notes Payable - Fixed Rate [Member] | New Louden Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		13,369
Effective interest rate	5.64%				5.64%
Mortgage Notes Payable - Fixed Rate [Member] | City Point [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		20,000
Effective interest rate	7.25%				7.25%
Mortgage Notes Payable - Fixed Rate [Member] | Crescent Plaza [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	16,455				16,455		16,747
Effective interest rate	4.98%				4.98%
Mortgage Notes Payable - Fixed Rate [Member] | Pacesetter Park Shopping Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	11,307				11,307		11,530
Effective interest rate	5.12%				5.12%
Mortgage Notes Payable - Fixed Rate [Member] | Elmwood Park Shopping Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	32,201				32,201		32,744
Effective interest rate	5.53%				5.53%
Mortgage Notes Payable - Fixed Rate [Member] | Chicago Street Retail Portfolio [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	15,265				15,265		15,558
Effective interest rate	5.61%				5.61%
Mortgage Notes Payable - Fixed Rate [Member] | The Gateway Shopping Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	19,440				19,440		19,746
Effective interest rate	5.44%				5.44%
Mortgage Notes Payable - Fixed Rate [Member] | 330 - 340 River Street [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	10,668				10,668		10,904
Effective interest rate	5.30%				5.30%
Mortgage Notes Payable - Fixed Rate [Member] | Brandywine Town Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	166,200				166,200		166,200
Effective interest rate	5.99%				5.99%
Mortgage Notes Payable - Fixed Rate [Member] | Walnut Hill Plaza [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		22,910
Effective interest rate	6.06%				6.06%
Mortgage Notes Payable - Fixed Rate [Member] | Rhode Island Place Shopping Center [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	15,975				15,975		16,208
Effective interest rate	6.35%				6.35%
Mortgage Notes Payable - Fixed Rate [Member] | 239 Greenwich Avenue [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	26,000				26,000		26,000
Effective interest rate	5.42%				5.42%
Mortgage Notes Payable - Fixed Rate [Member] | 639 West Diversey [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	4,245				4,245		4,341
Effective interest rate	6.65%				6.65%
Mortgage Notes Payable - Fixed Rate [Member] | Merrillville Plaza [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	25,504				25,504		25,837
Effective interest rate	5.88%				5.88%
Mortgage Notes Payable - Fixed Rate [Member] | Bedford Green [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	29,586				29,586		0
Effective interest rate	5.10%				5.10%
Mortgage Notes Payable - Fixed Rate [Member] | 216th Street [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	0				0		25,500
Effective interest rate	5.80%				5.80%
Mortgage Notes Payable - Fixed Rate [Member] | City Point, 1% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	5,262				5,262		5,262
Effective interest rate	1.00%				1.00%
Mortgage Notes Payable - Fixed Rate [Member] | City Point, 4.75% [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	199,000	[3]			199,000	[3]	197,000	[3]
Effective interest rate	4.75%	[3]			4.75%	[3]
Mortgage loan maturity					5 years
Convertible Notes [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable	380	[4]			380	[4]	$ 380	[4]
Effective interest rate	3.75%	[4]			3.75%	[4]			6.03%

[1] The Fund IV revolving subscription line of credit is secured by unfunded investor capital commitments.

[2] Represents the amount of the Company’s variable-rate debt that has been fixed through certain cash flow hedge transactions (Note 10).

[3] The maturity date of this loan is five years after the final advancing of funds which is currently anticipated to occur by the end of 2015.

[4] Holders of the Convertible Notes may require the Company to repurchase them at par on December 15, 2016 and December 15, 2021. The Company may redeem the Convertible Notes, in whole, or in part, at any time