Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)				3 Months Ended	6 Months Ended
		May 05, 2016 USD ($)	Apr. 22, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) property item	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Principal balance outstanding				$ 2,256,955,000	$ 2,256,955,000		$ 2,145,393,000
Repayment of mortgages, loans payable and other obligations					143,322,000	$ 27,251,000
Gain from extinguishment of debt				$ 12,420,000	$ 12,420,000
Minimum [Member]
Debt Instrument [Line Items]
Percentage of interest in venture				7.50%	7.50%
Maximum [Member]
Debt Instrument [Line Items]
Percentage of interest in venture				85.00%	85.00%
Chase II Project [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 48,000,000	$ 48,000,000
Port Imperial South 11 [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				78,000,000	78,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Principal balance outstanding				774,798,000	774,798,000		731,624,000
Adjustment for unamortized debt discount							(548,000)
Unamortized deferred financing costs				(5,375,000)	(5,375,000)		(4,465,000)
Total mortgages, loans payable and other obligations, net				769,423,000	$ 769,423,000		726,611,000
Secured Debt [Member] | Port Imperial South [Member]
Debt Instrument [Line Items]
Property Name	[1]				Port Imperial South
Lender	[1]				Wells Fargo Bank N.A.
LIBOR	[1],[2]				LIBOR+1.75
Spread over LIBOR					1.75%
Principal balance outstanding	[1]						34,962,000
Secured Debt [Member] | 6 Becker, 85 Livingston, 75 Livingston & 20 Waterview [Member]
Debt Instrument [Line Items]
Property Name	[3]				6 Becker, 85 Livingston, 75 Livingston & 20 Waterview
Lender	[3]				Wells Fargo CMBS
Effective rate	[2],[3]			10.26%	10.26%
Principal balance outstanding	[3]						63,279,000
Repayment of mortgages, loans payable and other obligations			$ 51,500,000
Gain from extinguishment of debt			$ 12,400,000
Secured Debt [Member] | 9200 Edmonston Road [Member]
Debt Instrument [Line Items]
Property Name	[4]				9200 Edmonston Road
Lender	[4]				Principal Commercial Funding L.L.C.
Effective rate	[2],[4]			9.78%	9.78%
Principal balance outstanding	[4]						3,793,000
Gain on sale		$ 200,000
Secured Debt [Member] | 4 Becker [Member]
Debt Instrument [Line Items]
Property Name					4 Becker
Lender					Wells Fargo CMBS
Effective rate	[2]			9.55%	9.55%
Principal balance outstanding				$ 40,330,000	$ 40,330,000		40,631,000
Loan maturity date	[5]				May 11, 2016
Secured Debt [Member] | Curtis Center [Member]
Debt Instrument [Line Items]
Property Name	[6]				Curtis Center
Lender	[6]				CCRE & PREFG
LIBOR	[2],[6],[7]				LIBOR+5.912
Spread over LIBOR	[7]				5.912%
Principal balance outstanding	[6]			$ 75,000,000	$ 75,000,000		64,000,000
Loan maturity date	[6]				Oct. 09, 2016
Percentage of interest in venture				50.00%	50.00%
Number of extension options | item					3
Loan extension period					1 year
Deferred financing costs amortization interest rate					1.362%
Secured Debt [Member] | Curtis Center [Member] | Senior Loan [Member]
Debt Instrument [Line Items]
Effective rate				3.7371%	3.7371%
Spread over LIBOR					3.29%
Principal balance outstanding				$ 102,000,000	$ 102,000,000
Percentage of interest in venture				50.00%	50.00%
LIBOR measurement period					1 month
Secured Debt [Member] | Curtis Center [Member] | Mezzanine Loan [Member]
Debt Instrument [Line Items]
Effective rate				9.943%	9.943%
Spread over LIBOR					9.50%
Principal balance outstanding				$ 48,000,000	$ 48,000,000
Percentage of interest in venture				50.00%	50.00%
LIBOR measurement period					1 month
Secured Debt [Member] | Various [Member]
Debt Instrument [Line Items]
Property Name	[8]				Various
Lender	[8]				Prudential Insurance
Effective rate	[2],[8]			6.332%	6.332%
Principal balance outstanding	[8]			$ 142,443,000	$ 142,443,000		143,513,000
Loan maturity date	[8]				Jan. 15, 2017
Number of properties used to collateralized mortgage | property					7
Repayment of mortgages, loans payable and other obligations					$ 61,100,000
Secured Debt [Member] | 150 Main St [Member]
Debt Instrument [Line Items]
Property Name	[9]				150 Main St.
Lender	[9]				Webster Bank
LIBOR	[2],[9]				LIBOR+2.35
Spread over LIBOR					2.35%
Principal balance outstanding	[9]			20,929,000	$ 20,929,000		10,937,000
Loan maturity date	[9]				Mar. 30, 2017
Secured Debt [Member] | 150 Main St [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 28,800,000	$ 28,800,000
Secured Debt [Member] | Portside 7 [Member]
Debt Instrument [Line Items]
Property Name	[10]				Portside 7
Lender	[10]				RBS Citizens N.A. & Salem Five Cents Savings Bank
Effective rate	[2],[10]			2.15%	2.15%
Spread over LIBOR					2.15%
Principal balance outstanding	[10]			$ 42,500,000	$ 42,500,000
Secured Debt [Member] | 23 Main Street [Member]
Debt Instrument [Line Items]
Property Name					23 Main Street
Lender					JPMorgan CMBS
Effective rate	[2]			5.587%	5.587%
Principal balance outstanding				$ 28,195,000	$ 28,195,000		28,541,000
Loan maturity date					Sep. 01, 2018
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial 4/5 Hotel
Lender					Fifth Third Bank & Santander
Effective rate	[2]			4.50%	4.50%
Spread over LIBOR					4.50%
Principal balance outstanding				$ 3,094,000	$ 3,094,000
Loan maturity date					Oct. 06, 2018
Secured Debt [Member] | Harborside Plaza 5 [Member]
Debt Instrument [Line Items]
Property Name					Harborside Plaza 5
Lender					The Northwestern Mutual Life Insurance Co. & New York Life Insurance Co.
Effective rate	[2]			6.842%	6.842%
Principal balance outstanding				$ 215,723,000	$ 215,723,000		217,736,000
Loan maturity date					Nov. 01, 2018
Secured Debt [Member] | Chase II Project [Member]
Debt Instrument [Line Items]
Property Name	[11]				Chase II (k)
Lender	[11]				Fifth Third Bank
Effective rate	[2],[11]			2.25%	2.25%
Spread over LIBOR					2.25%
Principal balance outstanding	[11]			$ 8,680,000	$ 8,680,000
Loan maturity date	[11]				Dec. 15, 2018
Secured Debt [Member] | 100 Walnut Avenue [Member]
Debt Instrument [Line Items]
Property Name					100 Walnut Avenue
Lender					Guardian Life Insurance Co.
Effective rate	[2]			7.311%	7.311%
Principal balance outstanding				$ 18,131,000	$ 18,131,000		18,273,000
Loan maturity date					Feb. 01, 2019
Secured Debt [Member] | One River Center [Member]
Debt Instrument [Line Items]
Property Name	[12]				One River Center
Lender	[12]				Guardian Life Insurance Co.
Effective rate	[2],[12]			7.311%	7.311%
Principal balance outstanding	[12]			$ 41,534,000	$ 41,534,000		41,859,000
Loan maturity date	[12]				Feb. 01, 2019
Number of properties used to collateralized mortgage | property					3
Secured Debt [Member] | Park Square [Member]
Debt Instrument [Line Items]
Property Name					Park Square
Lender					Wells Fargo Bank N.A.
LIBOR	[2],[13]				LIBOR+1.872
Spread over LIBOR	[13]				1.872%
Principal balance outstanding				$ 27,500,000	$ 27,500,000		27,500,000
Loan maturity date					Apr. 10, 2019
Deferred financing costs amortization interest rate					0.122%
Secured Debt [Member] | Port Imperial South 11 [Member]
Debt Instrument [Line Items]
Property Name	[14]				Port Imperial South 11
Lender	[14]				JPMorgan Chase
Effective rate	[2],[14]			2.35%	2.35%
Spread over LIBOR					2.35%
Principal balance outstanding	[14]			$ 1,639,000	$ 1,639,000
Loan maturity date	[14]				Nov. 24, 2019
Secured Debt [Member] | Port Imperial South 4/5 Retail [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial South 4/5 Retail
Lender					American General Life & A/G PC
Effective rate	[2]			4.559%	4.559%
Principal balance outstanding				$ 4,000,000	$ 4,000,000		4,000,000
Loan maturity date					Dec. 01, 2021
Secured Debt [Member] | The Chase At Overlook Ridge [Member]
Debt Instrument [Line Items]
Property Name					The Chase at Overlook Ridge
Lender					New York Community Bank
Effective rate	[2]			3.74%	3.74%
Principal balance outstanding				$ 72,500,000	$ 72,500,000
Loan maturity date					Feb. 01, 2023
Secured Debt [Member] | Port Imperial 4/5 Garage Development [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial South 4/5 Garage
Lender					American General Life & A/G PC
Effective rate	[2]			4.853%	4.853%
Principal balance outstanding				$ 32,600,000	$ 32,600,000		$ 32,600,000
Loan maturity date					Dec. 01, 2029

[1] On January 19, 2016, the loan was repaid in full at maturity, using borrowings from the Company's revolving credit facility.

[2] Reflects effective rate of debt, including deferred financing costs, comprised of the cost of terminated treasury lock agreements (if any), debt initiation costs, mark-to-market adjustment of acquired debt and other transaction costs, as applicable.

[3] On April 22, 2016, the loan was repaid at a discount for $51.5 million, using borrowings from the Company's revolving credit facility. Accordingly, the Company recognized a gain on extinguishment of debt of $12.4 million.

[4] On May 5, 2016, the Company transferred the deed for 9200 Edmonston Road to the lender in satisfaction of its obligations and recorded a gain of $0.2 million.

[5] The Company has begun discussions with the lender regarding the past due maturity of the loan.

[6] The Company owns a 50 percent tenants-in-common interest in the Curtis Center property. The Company's $75 million loan consists of its 50 percent interest in a $102 million senior loan with a current rate of 3.7371 percent at June 30, 2016 and its 50 percent interest in a $48 million mezzanine loan with a current rate of 9.943 percent at June 30, 2016. The senior loan rate is based on a floating rate of one-month LIBOR plus 329 basis points and the mezzanine loan rate is based on a floating rate of one-month LIBOR plus 950 basis points. The Company has entered into LIBOR caps for the periods of the loans. The loans provide for three one-year extension options.

[7] The effective interest rate includes amortization of deferred financing costs of 1.362 percent.

[8] Mortgage is cross collateralized by seven properties. The Company has agreed, subject to certain conditions, to guarantee repayment of $61.1 million of the loan.

[9] This construction loan has a maximum borrowing capacity of $28.8 million.

[10] On July 8, 2016, the loan was repaid in full from loan refinancing proceeds.

[11] This construction loan has a maximum borrowing capacity of $48 million.

[12] Mortgage is collateralized by the three properties comprising One River Center.

[13] The effective interest rate includes amortization of deferred financing costs of 0.122 percent.

[14] This constuction loan has a maximum borrowing capacity of $78 million.