Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)
 3 Months Ended
Mar. 29, 2018
USD ($)
Mar. 01, 2018
USD ($)
Jan. 08, 2018
USD ($)
Mar. 31, 2018
USD ($)
property
item
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]            
Principal balance outstanding       $ 2,615,211,000   $ 2,809,568,000
Borrowings from revolving credit facility       322,000,000 $ 275,000,000  
Payment for borrowings       281,000,000 $ 471,000,000  
Secured Debt [Member]            
Debt Instrument [Line Items]            
Principal balance outstanding       1,189,614,000   1,426,111,000
Unamortized deferred financing costs       (7,579,000)   (7,976,000)
Total mortgages, loans payable and other obligations, net       $ 1,182,035,000   1,418,135,000
Secured Debt [Member] | Harborside Plaza 5 [Member]            
Debt Instrument [Line Items]            
Property Name [1]       Harborside Plaza 5    
Lender [1]       The Northwestern Mutual Life Insurance Co. & New York Life Insurance Co.    
Effective rate [1],[2]       6.84%    
Principal balance outstanding [1]         209,257,000
Payment for borrowings     $ 8,400,000      
Secured Debt [Member] | 23 Main Street [Member]            
Debt Instrument [Line Items]            
Property Name [3]       23 Main Street    
Lender [3]       Berkadia CMBS    
Effective rate [2],[3]       5.59%    
Principal balance outstanding [3]         27,090,000
Payment for borrowings   $ 100,000        
Secured Debt [Member] | One River Center [Member]            
Debt Instrument [Line Items]            
Property Name [4]       One River Center    
Lender [4]       Guardian Life Insurance Co.    
Effective rate [2],[4]       7.31%    
Principal balance outstanding [4]         40,485,000
Number of properties used to collateralized mortgage | property       3    
Payment for borrowings $ 1,800,000          
Secured Debt [Member] | Park Square [Member]            
Debt Instrument [Line Items]            
Property Name       Park Square    
Lender       Wells Fargo Bank N.A.    
LIBOR [2]       LIBOR+1.87    
Spread over LIBOR       1.87%    
Principal balance outstanding       $ 26,217,000   26,567,000
Loan maturity date       Apr. 10, 2019    
Secured Debt [Member] | 250 Johnson Road [Member]            
Debt Instrument [Line Items]            
Property Name [5]       250 Johnson    
Lender [5]       M&T Bank    
LIBOR [2],[5]       LIBOR+2.35    
Spread over LIBOR [5]       2.35%    
Principal balance outstanding [5]       $ 37,028,000   32,491,000
Loan maturity date [5]       May 20, 2019    
Secured Debt [Member] | 250 Johnson Road [Member] | Construction Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 42,000,000    
Number of extension options | item       1    
Loan extension period       1 year    
Extension fee       0.25%    
Secured Debt [Member] | Portside 5/6 [Member]            
Debt Instrument [Line Items]            
Property Name [6]       Portside 5/6    
Lender [6]       Citizens Bank    
LIBOR [2],[6]       LIBOR+2.50    
Spread over LIBOR [6]       2.50%    
Principal balance outstanding [6]       $ 56,541,000   45,778,000
Loan maturity date [6]       Sep. 29, 2019    
Secured Debt [Member] | Portside 5/6 [Member] | Construction Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 73,000,000    
Number of extension options | item       2    
Loan extension period       1 year    
Extension fee       0.15%    
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 [2],[7]       LIBOR+4.50    
Spread over LIBOR [7]       4.50%    
Principal balance outstanding [7]       $ 50,958,000   43,674,000
Loan maturity date [7]       Oct. 06, 2019    
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member] | Construction Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 94,000,000    
Number of extension options | item       2    
Loan extension period       1 year    
Extension fee       0.20%    
Secured Debt [Member] | Port Imperial South 11 [Member]            
Debt Instrument [Line Items]            
Property Name [8]       Port Imperial South 11    
Lender [8]       JPMorgan Chase    
LIBOR [2],[8]       LIBOR+2.35    
Spread over LIBOR [8]       2.35%    
Principal balance outstanding [8]       $ 54,341,000   46,113,000
Loan maturity date [8]       Nov. 24, 2019    
Secured Debt [Member] | Port Imperial South 11 [Member] | Construction Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 78,000,000    
Number of extension options | item       2    
Loan extension period       1 year    
Extension fee       0.15%    
Secured Debt [Member] | Worcester [Member]            
Debt Instrument [Line Items]            
Property Name [9]       Worcester    
Lender [9]       Citizens Bank    
LIBOR [2],[9]       LIBOR+2.50    
Spread over LIBOR [9]       2.50%    
Principal balance outstanding [9]       $ 48,099,000   37,821,000
Loan maturity date [9]       Dec. 10, 2019    
Secured Debt [Member] | Worcester [Member] | Construction Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 58,000,000    
Number of extension options | item       2    
Loan extension period       1 year    
Extension fee       0.15%    
Secured Debt [Member] | Monaco [Member]            
Debt Instrument [Line Items]            
Property Name [10]       Monaco    
Lender [10]       The Northwestern Mutual Life Insurance Co.    
Effective rate [2],[10]       3.15%    
Principal balance outstanding [10]       $ 169,582,000   169,987,000
Loan maturity date [10]       Feb. 01, 2021    
Adjustment for unamortized debt discount       $ 5,000,000    
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 [2]       4.56%    
Principal balance outstanding       $ 4,000,000   4,000,000
Loan maturity date       Dec. 01, 2021    
Secured Debt [Member] | Portside 7 [Member]            
Debt Instrument [Line Items]            
Property Name       Portside 7    
Lender       CBRE Capital Markets/FreddieMac    
Effective rate [2]       3.57%    
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 [2]       3.85%    
Principal balance outstanding       $ 100,000,000   100,000,000
Loan maturity date       Feb. 01, 2024    
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%    
Principal balance outstanding       $ 135,750,000   135,750,000
Loan maturity date       Jan. 01, 2025    
Secured Debt [Member] | 101 Hudson Street [Member]            
Debt Instrument [Line Items]            
Property Name       101 Hudson    
Lender       Wells Fargo CMBS    
Effective rate [2]       3.20%    
Principal balance outstanding       $ 250,000,000   250,000,000
Loan maturity date       Oct. 11, 2026    
Secured Debt [Member] | Short Hills Portfolio [Member]            
Debt Instrument [Line Items]            
Property Name [11]       Short Hills Portfolio    
Lender [11]       Wells Fargo CMBS    
Effective rate [2],[11]       4.15%    
Principal balance outstanding [11]       $ 124,500,000   124,500,000
Loan maturity date [11]       Apr. 01, 2027    
Secured Debt [Member] | 150 Main St [Member]            
Debt Instrument [Line Items]            
Property Name       150 Main St.    
Lender       Natixis Real Estate Capital LLC    
Effective rate [2]       4.48%    
Principal balance outstanding       $ 41,000,000   41,000,000
Loan maturity date       Aug. 05, 2027    
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]       4.85%    
Principal balance outstanding       $ 32,600,000   $ 32,600,000
Loan maturity date       Dec. 01, 2029    
[1] On January 8, 2018, the Company prepaid this loan in full upon payment of a fee of approximately $8.4 million using borrowings from the Company's unsecured 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 March 1, 2018, the Company prepaid this loan in full upon payment of a fee of approximately $0.1 million using borrowings from the Company's unsecured revolving credit facility.
[4] Mortgage was collateralized by the three properties comprising One River Center. On March 29, 2018, the Company prepaid this loan in full upon payment of a fee of approximately $1.8 million using borrowings from the Company's unsecured revolving credit facility.
[5] This construction loan has a maximum borrowing capacity of $42 million and provides, subject to certain conditions, a one-year extension option with a fee of 25 basis points. See Note 12: Commitments and Contingencies - Construction Projects.
[6] This construction loan has a maximum borrowing capacity of $73 million and provides, subject to certain conditions, two one-year extension options with a fee of 15 basis points each year. See Note 12: Commitments and Contingencies - Construction Projects.
[7] This construction loan has a maximum borrowing capacity of $94 million and provides, subject to certain conditions, two one-year extension options with a fee of 20 basis points for each year. See Note 12: Commitments and Contingencies - Construction Projects.
[8] This construction loan has a maximum borrowing capacity of $78 million and provides, subject to certain conditions, two one-year extension options with a fee of 15 basis points each year. See Note 12: Commitments and Contingencies - Construction Projects
[9] This construction loan has a maximum borrowing capacity of $58 million and provides, subject to certain conditions, two one-year extension options with a fee of 15 basis points each year. See Note 12: Commitments and Contingencies - Construction Projects.
[10] This mortgage loan, which includes unamortized fair value adjustment of $5.0 million as of March 31, 2018, was assumed by the Company in April 2017 with the consolidation of all the interests in Monaco Towers.
[11] This mortgage loan was obtained by the Company in March 2017 to partially fund the acquisition of the Short Hills/Madison portfolio.