Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
property
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 29, 2017
USD ($)
Debt Instrument [Line Items]        
Principal balance outstanding $ 2,809,568,000 $ 2,340,009,000    
Adjustment for unamortized debt discount 3,505,000      
Secured Debt 1,418,135,000 888,585,000    
Borrowings from revolving credit facility $ 730,000,000 1,165,000,000 $ 334,000,000  
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]        
Number of extension options | item 2      
Loan extension period 1 year      
Extension fee 0.20%      
250 Johnson Road [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Number of extension options | item 1      
Loan extension period 1 year      
Extension fee 0.25%      
Portside 5/6 [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Number of extension options | item 2      
Loan extension period 1 year      
Extension fee 0.15%      
Port Imperial South 11 [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Number of extension options | item 2      
Loan extension period 1 year      
Extension fee 0.15%      
Worcester [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Number of extension options | item 2      
Loan extension period 1 year      
Extension fee 0.15%      
Secured Debt [Member]        
Debt Instrument [Line Items]        
Principal balance outstanding $ 1,426,111,000 896,055,000    
Unamortized deferred financing costs (7,976,000) (7,470,000)    
Total mortgages, loans payable and other obligations, net $ 1,418,135,000 888,585,000    
Secured Debt [Member] | Curtis Center [Member]        
Debt Instrument [Line Items]        
Property Name [1] Curtis Center      
Lender [1] CCRE & PREFG      
LIBOR [1],[2] LIBOR+5.91      
Spread over LIBOR [1] 5.91%      
Principal balance outstanding   75,000,000 [1]   $ 75,000,000
Percentage of interest in venture 50.00%      
Secured Debt [Member] | Chase II [Member]        
Debt Instrument [Line Items]        
Property Name [3] Chase II      
Lender [3] Fifth Third Bank      
LIBOR [2],[3] LIBOR+2.25      
Spread over LIBOR [3] 2.25%      
Principal balance outstanding [3] 34,708,000    
Secured Debt [Member] | 23 Main Street [Member]        
Debt Instrument [Line Items]        
Property Name 23 Main Street      
Lender Berkadia CMBS      
Effective rate [2] 5.59%      
Principal balance outstanding $ 27,090,000 27,838,000    
Loan maturity date Sep. 01, 2018      
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member]        
Debt Instrument [Line Items]        
Property Name [4] Port Imperial 4/5 Hotel      
Lender [4] Fifth Third Bank & Santander      
LIBOR [2],[4] LIBOR+4.50      
Spread over LIBOR [4] 4.50%      
Principal balance outstanding [4] $ 43,674,000 14,919,000    
Loan maturity date [4] Oct. 06, 2018      
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 94,000,000      
Secured Debt [Member] | Harborside Plaza 5 [Member]        
Debt Instrument [Line Items]        
Property Name [5] Haborside Plaza 5      
Lender [5] The Northwestern Mutual Life Insurance Co. & New York Life Insurance Co.      
Effective rate [2],[5] 6.84%      
Principal balance outstanding [5] $ 209,257,000 213,640,000    
Loan maturity date [5] Nov. 01, 2018      
Borrowings from revolving credit facility $ 8,400,000      
Secured Debt [Member] | One River Center [Member]        
Debt Instrument [Line Items]        
Property Name [6] One River Center      
Lender [6] Guardian Life Insurance Co.      
Effective rate [2],[6] 7.31%      
Principal balance outstanding [6] $ 40,485,000 41,197,000    
Loan maturity date [6] 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] LIBOR+1.87      
Spread over LIBOR 1.87%      
Principal balance outstanding $ 26,567,000 27,500,000    
Loan maturity date Apr. 10, 2019      
Secured Debt [Member] | 250 Johnson Road [Member]        
Debt Instrument [Line Items]        
Property Name [7] 250 Johnson      
Lender [7] M&T Bank      
LIBOR [2],[7] LIBOR+2.35      
Spread over LIBOR [7] 2.35%      
Principal balance outstanding [7] $ 32,491,000 2,440,000    
Loan maturity date [7] May 20, 2019      
Secured Debt [Member] | 250 Johnson Road [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 42,000,000      
Secured Debt [Member] | Portside 5/6 [Member]        
Debt Instrument [Line Items]        
Property Name [8] Portside 5/6      
Lender [8] Citizens Bank      
LIBOR [2],[8] LIBOR+2.50      
Spread over LIBOR [8] 2.50%      
Principal balance outstanding [8] $ 45,778,000      
Loan maturity date [8] Sep. 29, 2019      
Secured Debt [Member] | Portside 5/6 [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 73,000,000      
Secured Debt [Member] | Port Imperial South 11 [Member]        
Debt Instrument [Line Items]        
Property Name [9] Port Imperial South 11      
Lender [9] JPMorgan Chase      
LIBOR [2],[9] LIBOR+2.35      
Spread over LIBOR [9] 2.35%      
Principal balance outstanding [9] $ 46,113,000 14,073,000    
Loan maturity date [9] Nov. 24, 2019      
Secured Debt [Member] | Port Imperial South 11 [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 78,000,000      
Secured Debt [Member] | Worcester [Member]        
Debt Instrument [Line Items]        
Property Name [10] Worcester      
Lender [10] Citizens Bank      
LIBOR [2],[10] LIBOR+2.50      
Spread over LIBOR [10] 2.50%      
Principal balance outstanding [10] $ 37,821,000      
Loan maturity date [10] Dec. 10, 2019      
Secured Debt [Member] | Worcester [Member] | Construction Loan [Member]        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 58,000,000      
Secured Debt [Member] | Monaco [Member]        
Debt Instrument [Line Items]        
Property Name [11] Monaco      
Lender [11] The Northwestern Mutual Life Insurance Co.      
Effective rate [2],[11] 3.15%      
Principal balance outstanding [11] $ 169,987,000      
Loan maturity date [11] 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] | 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 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 [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      
Loan maturity date Feb. 01, 2024      
Secured Debt [Member] | The Chase At Overlook Ridge And Chase II [Member]        
Debt Instrument [Line Items]        
Property Name The Chase at Overlook Ridge and Chase II      
Lender New York Community Bank      
Effective rate [2] 3.74%      
Principal balance outstanding $ 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 [12] Short Hills Portfolio      
Lender [12] Wells Fargo CMBS      
Effective rate [2],[12] 4.15%      
Principal balance outstanding [12] $ 124,500,000      
Loan maturity date [12] 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 26,642,000    
Loan maturity date Aug. 05, 2027      
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 [2] 4.85%      
Principal balance outstanding $ 32,600,000 $ 32,600,000    
Loan maturity date Dec. 01, 2029      
[1] The Company owned a 50 percent tenants-in-common interest in the Curtis Center property. On September 29, 2017, the Company sold its equity interest to its joint venture partner, which included the retirement of this $75 million loan balance.
[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] The Chase II construction loan was paid off on December 5, 2017 using the proceeds of a new combined mortgage loan secured by The Chase at Overlook Ridge and Chase II.
[4] 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.
[5] 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.
[6] Mortgage is collateralized by the three properties comprising One River Center.
[7] 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.
[8] 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.
[9] 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.
[10] 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.
[11] This mortgage loan, which includes unamortized fair value adjustment of $5.0 million as of December 31, 2017, was assumed by the Company in April 2017 with the consolidation of all the interests in Monaco Towers. See Note 3: Recent Transactions - Consolidations.
[12] This mortgage loan was obtained by the Company in March 2017 to partially fund the acquisition of the Short Hills/Madison portfolio.