Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)
 3 Months Ended
Mar. 31, 2017
USD ($)
property
item
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]    
Principal balance outstanding $ 2,731,204,000 $ 2,340,009,000
Secured Debt $ 1,152,443,000 888,585,000
Minimum [Member]    
Debt Instrument [Line Items]    
Percentage of interest in venture 12.50%  
Maximum [Member]    
Debt Instrument [Line Items]    
Percentage of interest in venture 85.00%  
Port Imperial 4/5 Hotel [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 94,000,000  
Chase II Project [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 48,000,000  
One River Center [Member]    
Debt Instrument [Line Items]    
Number of properties used to collateralized mortgage | property 3  
250 Johnson Road [Member]    
Debt Instrument [Line Items]    
Loan maturity date [1] May 20, 2019  
250 Johnson Road [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 42,000,000  
Portside 5/6 Project [Member]    
Debt Instrument [Line Items]    
Loan maturity date [2] Sep. 19, 2019  
Portside 5/6 Project [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 73,000,000  
Port Imperial South 11 [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 78,000,000  
Worcester [Member]    
Debt Instrument [Line Items]    
Loan maturity date [3] Dec. 10, 2019  
Worcester [Member] | Construction Loan [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 58,000,000  
Alterra I & II [Member]    
Debt Instrument [Line Items]    
Loan maturity date Feb. 01, 2024  
Short Hills Office Buildings [Member]    
Debt Instrument [Line Items]    
Loan maturity date [4] Apr. 01, 2027  
Secured Debt [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 1,161,244,000 896,055,000
Unamortized deferred financing costs (8,801,000) (7,470,000)
Total mortgages, loans payable and other obligations, net $ 1,152,443,000 888,585,000
Secured Debt [Member] | 150 Main St [Member]    
Debt Instrument [Line Items]    
Property Name 150 Main St.  
Lender Webster Bank  
LIBOR [5] LIBOR+2.35  
Spread over LIBOR 2.35%  
Principal balance outstanding $ 28,540,000 26,642,000
Loan maturity date Aug. 01, 2017  
Secured Debt [Member] | Curtis Center [Member]    
Debt Instrument [Line Items]    
Property Name [6] Curtis Center  
Lender [6] CCRE & PREFG  
LIBOR [5],[6] LIBOR+5.912  
Spread over LIBOR [6] 5.912%  
Principal balance outstanding [6] $ 75,000,000 75,000,000
Loan maturity date [6] Oct. 09, 2017  
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 4.207%  
Spread over LIBOR 3.29%  
Principal balance outstanding $ 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]    
Effective rate 10.413%  
Spread over LIBOR 9.50%  
Principal balance outstanding $ 48,000,000  
Percentage of interest in venture 50.00%  
LIBOR measurement period 1 month  
Secured Debt [Member] | 23 Main Street [Member]    
Debt Instrument [Line Items]    
Property Name 23 Main Street  
Lender Berkadia CMBS  
Effective rate [5] 5.587%  
Principal balance outstanding $ 27,650,000 27,838,000
Loan maturity date Sep. 01, 2018  
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member]    
Debt Instrument [Line Items]    
Property Name [7] Port Imperial 4/5 Hotel  
Lender [7] Fifth Third Bank & Santander  
LIBOR [5],[7] LIBOR+4.50  
Spread over LIBOR [7] 4.50%  
Principal balance outstanding [7] $ 24,530,000 14,919,000
Loan maturity date [7] 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 [5] 6.842%  
Principal balance outstanding $ 212,572,000 213,640,000
Loan maturity date Nov. 01, 2018  
Secured Debt [Member] | Chase II Project [Member]    
Debt Instrument [Line Items]    
Property Name [8] Chase II  
Lender [8] Fifth Third Bank  
LIBOR [5],[8] LIBOR+2.25  
Spread over LIBOR [8] 2.25%  
Principal balance outstanding [8] $ 40,317,000 34,708,000
Loan maturity date [8] Dec. 16, 2018  
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 [5],[9] 7.311%  
Principal balance outstanding [9] $ 41,024,000 41,197,000
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 [5] LIBOR+1.872  
Spread over LIBOR 1.872%  
Principal balance outstanding $ 27,500,000 27,500,000
Loan maturity date Apr. 10, 2019  
Secured Debt [Member] | 250 Johnson Road [Member]    
Debt Instrument [Line Items]    
Property Name [1] 250 Johnson  
Lender [1] M&T Bank  
LIBOR [1],[5] LIBOR+2.35  
Spread over LIBOR [1] 2.35%  
Principal balance outstanding [1] $ 6,147,000 2,440,000
Secured Debt [Member] | Portside 5/6 Project [Member]    
Debt Instrument [Line Items]    
Property Name [2] Portside 5/6  
Lender [2] Citizens Bank  
LIBOR [2],[5] LIBOR+2.50  
Spread over LIBOR [2] 2.50%  
Principal balance outstanding [2] $ 8,084,000  
Secured Debt [Member] | Port Imperial South 11 [Member]    
Debt Instrument [Line Items]    
Property Name [10] Port Imperial South 11  
Lender [10] JPMorgan Chase  
LIBOR [5],[10] LIBOR+2.35  
Spread over LIBOR [10] 2.35%  
Principal balance outstanding [10] $ 22,246,000 14,073,000
Loan maturity date [10] Nov. 24, 2019  
Secured Debt [Member] | Worcester [Member]    
Debt Instrument [Line Items]    
Property Name [3] Worcester  
Lender [3] Citizens Bank  
LIBOR [3],[5] LIBOR+2.50  
Spread over LIBOR [3] 2.50%  
Principal balance outstanding [3] $ 5,036,000  
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 [5] 4.559%  
Principal balance outstanding $ 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 [5] 3.74%  
Principal balance outstanding $ 72,500,000 72,500,000
Loan maturity date Feb. 01, 2023  
Secured Debt [Member] | Portside 7 [Member]    
Debt Instrument [Line Items]    
Property Name Portside 7  
Lender CBRE Capital Markets/FreddieMac  
Effective rate [5] 3.569%  
Principal balance outstanding $ 58,998,000 58,998,000
Loan maturity date Aug. 01, 2023  
Secured Debt [Member] | Alterra I & II [Member]    
Debt Instrument [Line Items]    
Property Name Alterra I & II  
Lender Capital One/FreddieMac  
Effective rate [5] 3.854%  
Principal balance outstanding $ 100,000,000  
Secured Debt [Member] | 101 Hudson Street [Member]    
Debt Instrument [Line Items]    
Property Name 101 Hudson  
Lender Wells Fargo CMBS  
Effective rate [5] 3.197%  
Principal balance outstanding $ 250,000,000 250,000,000
Loan maturity date Oct. 11, 2026  
Secured Debt [Member] | Short Hills Office Buildings [Member]    
Debt Instrument [Line Items]    
Property Name [4] Short Hills Office Buildings  
Lender [4] Wells Fargo CMBS  
Effective rate [4],[5] 4.149%  
Principal balance outstanding [4] $ 124,500,000  
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 [5] 4.853%  
Principal balance outstanding $ 32,600,000 $ 32,600,000
Loan maturity date Dec. 01, 2029  
[1] This construction loan has a maximum borrowing capacity of $42 million.
[2] This construction loan has a maximum borrowing capacity of $73 million.
[3] This construction loan has a maximum borrowing capacity of $58 million.
[4] This mortgage loan was obtained by the Company in March 2017 to partially fund the acquisition of the Short Hills/Madison portfolio.
[5] 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.
[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 4.207 percent at March 31, 2017 and its 50 percent interest in a $48 million mezzanine loan with a current rate of 10.413 percent at March 31, 2017. 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. In October 2016, the first of three one-year extension options was exercised by the venture.
[7] This construction loan has a maximum borrowing capacity of $94 million.
[8] This construction loan has a maximum borrowing capacity of $48 million.
[9] Mortgage is collateralized by the three properties comprising One River Center.
[10] This construction loan has a maximum borrowing capacity of $78 million.