Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)				3 Months Ended
		Apr. 22, 2016 USD ($)	Jan. 07, 2016 item	Mar. 31, 2016 USD ($) item property	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Loan balance				$ 2,269,287,000		$ 2,145,393,000
Number of extension options | item			2
Loan extension period			1 year
Repayment of mortgages, loans payable, and other obligations				$ 89,712,000	$ 25,228,000
Port Imperial South [Member]
Debt Instrument [Line Items]
Loan maturity date	[1]			Jan. 17, 2016
Curtis Center [Member]
Debt Instrument [Line Items]
Deferred financing costs amortization interest rate				1.362%
Curtis Center [Member] | Mezzanine Loan [Member]
Debt Instrument [Line Items]
Effective rate				9.937%
Loan balance				$ 26,000,000
Maximum borrowing capacity				$ 48,000,000
Various [Member]
Debt Instrument [Line Items]
Number of properties used to collateralize mortgage | property				7
Repayment of mortgages, loans payable, and other obligations				$ 61,100,000
150 Main St. [Member]
Debt Instrument [Line Items]
Loan maturity date	[2]			Mar. 30, 2017
150 Main St. [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 28,800,000
One River Center [Member]
Debt Instrument [Line Items]
Number of properties used to collateralize mortgage | property				3
Park Square [Member]
Debt Instrument [Line Items]
Deferred financing costs amortization interest rate				0.122%
Secured Debt [Member]
Debt Instrument [Line Items]
Loan balance				$ 772,241,000		731,624,000
Adjustment for unamortized debt discount				(222,000)		(548,000)
Unamortized deferred financing costs				(4,446,000)		(4,465,000)
Total mortgages, loans payable and other obligations, net				$ 767,573,000		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],[3]			LIBOR+1.75
Spread over LIBOR	[1]			1.75%
Loan balance	[1]					34,962,000
Secured Debt [Member] | 6 Becker, 85 Livingston, 75 Livingston & 20 Waterview [Member]
Debt Instrument [Line Items]
Property Name				6 Becker, 85 Livingston, 75 Livingston & 20 Waterview
Lender				Wells Fargo CMBS
Effective rate	[3]			10.26%
Loan balance				$ 63,279,000		63,279,000
Loan maturity date	[4]			Aug. 11, 2014
Number of properties used to collateralize mortgage | property				4
Secured Debt [Member] | 9200 Edmonston Road [Member]
Debt Instrument [Line Items]
Property Name				9200 Edmonston Road
Lender				Principal Commercial Funding L.L.C.
Effective rate	[3]			9.78%
Loan balance				$ 3,793,000		3,793,000
Loan maturity date	[5]			May 01, 2015
Secured Debt [Member] | 4 Becker [Member]
Debt Instrument [Line Items]
Property Name				4 Becker
Lender				Wells Fargo CMBS
Effective rate	[3]			9.55%
Loan balance				$ 40,478,000		40,631,000
Loan maturity date				May 11, 2016
Secured Debt [Member] | Curtis Center [Member]
Debt Instrument [Line Items]
Property Name	[6]			Curtis Center
Lender	[6]			CCRE & PREFG
LIBOR	[3],[6],[7]			LIBOR+5.912
Spread over LIBOR	[6]			5.912%
Loan balance	[6]			$ 64,000,000		64,000,000
Loan maturity date	[6]			Oct. 09, 2016
Percentage of interest in venture				50.00%
Number of extension options | item				3
Loan extension period				1 year
Secured Debt [Member] | Curtis Center [Member] | Senior Loan [Member]
Debt Instrument [Line Items]
Effective rate				3.7311%
Spread over LIBOR				3.29%
Loan balance				$ 102,000,000
Percentage of interest in venture				50.00%
LIBOR measurement period				1 month
Secured Debt [Member] | Curtis Center [Member] | Mezzanine Loan [Member]
Debt Instrument [Line Items]
Spread over LIBOR				9.50%
Percentage of interest in venture				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	[3],[8]			6.332%
Loan balance	[8]			$ 142,983,000		143,513,000
Loan maturity date	[8]			Jan. 15, 2017
Secured Debt [Member] | 150 Main St. [Member]
Debt Instrument [Line Items]
Property Name	[2]			150 Main St.
Lender	[2]			Webster Bank
LIBOR	[2],[3]			LIBOR+2.35
Spread over LIBOR	[2]			2.35%
Loan balance	[2]			$ 16,103,000		10,937,000
Secured Debt [Member] | 23 Main Street [Member]
Debt Instrument [Line Items]
Property Name				23 Main Street
Lender				JPMorgan CMBS
Effective rate	[3]			5.587%
Loan balance				$ 28,367,000		28,541,000
Loan maturity date				Sep. 01, 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	[3]			6.842%
Loan balance				$ 216,738,000		217,736,000
Loan maturity date				Nov. 01, 2018
Secured Debt [Member] | 100 Walnut Avenue [Member]
Debt Instrument [Line Items]
Property Name				100 Walnut Avenue
Lender				Guardian Life Insurance Co.
Effective rate	[3]			7.311%
Loan balance				$ 18,202,000		18,273,000
Loan maturity date				Feb. 01, 2019
Secured Debt [Member] | One River Center [Member]
Debt Instrument [Line Items]
Property Name	[9]			One River Center
Lender	[9]			Guardian Life Insurance Co.
Effective rate	[3],[9]			7.311%
Loan balance	[9]			$ 41,698,000		41,859,000
Loan maturity date	[9]			Feb. 01, 2019
Secured Debt [Member] | Park Square [Member]
Debt Instrument [Line Items]
Property Name				Park Square
Lender				Wells Fargo Bank N.A.
LIBOR	[3],[10]			LIBOR+1.872
Spread over LIBOR	[10]			1.872%
Loan balance				$ 27,500,000		27,500,000
Loan maturity date				Apr. 10, 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	[3]			4.559%
Loan balance				$ 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	[3]			3.74%
Loan balance				$ 72,500,000
Loan maturity date				Feb. 01, 2023
Secured Debt [Member] | Port Imperial South 4/5 Garage [Member]
Debt Instrument [Line Items]
Property Name				Port Imperial South 4/5 Garage
Lender				American General Life & A/G PC
Effective rate	[3]			4.853%
Loan balance				$ 32,600,000		$ 32,600,000
Loan maturity date				Dec. 01, 2029
Subsequent Event [Member] | Secured Debt [Member] | 6 Becker, 85 Livingston, 75 Livingston & 20 Waterview [Member]
Debt Instrument [Line Items]
Repayment of mortgages, loans payable, and other obligations		$ 51,500,000
Minimum [Member]
Debt Instrument [Line Items]
Percentage of interest in venture				7.50%
Maximum [Member]
Debt Instrument [Line Items]
Percentage of interest in venture				85.00%

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

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

[3] 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.

[4] Mortgage is cross collateralized by the four properties. On April 22, 2016, the loan was repaid for $51.5 million.

[5] Excess cash flow, as defined, is being held by the lender for re-leasing costs. The deed for the property was placed in escrow and is available to the lender in the event of default or non-payment at maturity. The mortgage loan was not repaid at maturity on May 1, 2015. The Company is in discussions with the lender regarding a further extension of the loan.

[6] The Company owns a 50 percent tenants-in-common interest in the Curtis Center property. The Company's $64.0 million loan consists of its 50 percent interest in a $102 million senior loan with a current rate of 3.7311 percent at March 31, 2016 and its 50 percent interest in a $26 million mezzanine loan (with a maximum borrowing capacity of $48 million) with a current rate of 9.937 percent at March 31, 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] Mortgage is collateralized by the three properties comprising One River Center.

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