Annual report pursuant to Section 13 and 15(d)

Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)

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Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)
 12 Months Ended
Dec. 17, 2018
USD ($)
Dec. 07, 2018
USD ($)
Mar. 29, 2018
USD ($)
Mar. 01, 2018
USD ($)
Jan. 08, 2018
USD ($)
Dec. 31, 2018
USD ($)
property
item
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]                
Principal balance outstanding           $ 2,792,651,000 $ 2,809,568,000  
Adjustment for unamortized debt discount           2,838,000    
Borrowings from revolving credit facility           461,000,000 730,000,000 $ 1,165,000,000
Payment for borrowings           $ 494,000,000 866,000,000 $ 1,034,000,000
250 Johnson Road [Member]                
Debt Instrument [Line Items]                
Loan maturity date           May 20, 2019    
Portside 5/6 [Member]                
Debt Instrument [Line Items]                
Loan maturity date           Mar. 10, 2026    
Secured Debt [Member]                
Debt Instrument [Line Items]                
Principal balance outstanding           $ 1,440,396,000 1,426,111,000  
Unamortized deferred financing costs           8,998,000 7,976,000  
Total mortgages, loans payable and other obligations, net           $ 1,431,398,000 1,418,135,000  
Secured Debt [Member] | Curtis Center [Member]                
Debt Instrument [Line Items]                
Property Name           Harborside Plaza    
Lender           The Northwestern Mutual Life Insurance & New York Life Insurance Co.    
Effective rate           6.84%    
Principal balance outstanding             209,257,000  
Secured Debt [Member] | Chase II [Member]                
Debt Instrument [Line Items]                
Lender           & New York Life Insurance Co.    
Principal balance outstanding              
Secured Debt [Member] | Harborside Plaza 5 [Member]                
Debt Instrument [Line Items]                
Payment for borrowings         $ 8,400,000      
Secured Debt [Member] | 23 Main Street [Member]                
Debt Instrument [Line Items]                
Property Name           23 Main Street    
Lender           Berkadia CMBS    
Effective rate           5.59%    
Principal balance outstanding           27,090,000  
Payment for borrowings       $ 100,000        
Secured Debt [Member] | One River Center [Member]                
Debt Instrument [Line Items]                
Property Name           One River Center    
Lender           Guardian Life Insurance Co.    
Effective rate           7.31%    
Principal balance outstanding           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           LIBOR+1.87    
Effective rate           1.87%    
Principal balance outstanding           $ 25,167,000 26,567,000  
Loan maturity date           Apr. 10, 2019    
Secured Debt [Member] | 250 Johnson Road [Member]                
Debt Instrument [Line Items]                
Property Name           250 Johnson    
Lender           M&T Bank    
LIBOR           LIBOR+2.35    
Effective rate           2.35%    
Principal balance outstanding           $ 41,769,000 32,491,000  
Secured Debt [Member] | 250 Johnson Road [Member] | Construction Loan [Member]                
Debt Instrument [Line Items]                
Maximum borrowing capacity           $ 42,000,000    
Loan extension period           1 year    
Extension fee           0.25%    
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    
LIBOR           LIBOR+4.50    
Effective rate           4.50%    
Principal balance outstanding           $ 73,350,000 43,674,000  
Loan maturity date           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] | Worcester [Member]                
Debt Instrument [Line Items]                
Property Name           Worcester    
Lender           Citizens Bank    
LIBOR           LIBOR+2.50    
Effective rate           2.50%    
Principal balance outstanding           $ 56,892,000 37,821,000  
Loan maturity date           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           Monaco    
Lender           The Northwestern Mutual Life Insurance Co.    
Effective rate           3.15%    
Principal balance outstanding           $ 168,370,000 169,987,000  
Loan maturity date           Feb. 01, 2021    
Adjustment for unamortized debt discount           $ 3,400,000    
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           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           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           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           3.74%    
Principal balance outstanding           $ 135,750,000 135,750,000  
Loan maturity date           Jan. 01, 2025    
Secured Debt [Member] | Portside 5/6 [Member]                
Debt Instrument [Line Items]                
Property Name           Portside 5/6    
Lender           New York Life Insurance Company    
Effective rate           4.56%    
Principal balance outstanding           $ 97,000,000 45,778,000  
Maximum borrowing capacity   $ 97,000,000            
Unamortized deferred financing costs   200,000            
Secured Debt [Member] | Portside 5/6 [Member] | Construction Loan [Member]                
Debt Instrument [Line Items]                
Payment for borrowings   $ 70,000,000            
Secured Debt [Member] | Marbella I [Member]                
Debt Instrument [Line Items]                
Property Name           Marbella    
Lender           New York Life Insurance Company    
Effective rate           4.17%    
Principal balance outstanding           $ 131,000,000    
Loan maturity date           Aug. 10, 2026    
Secured Debt [Member] | 101 Hudson [Member]                
Debt Instrument [Line Items]                
Property Name           101 Hudson    
Lender           Wells Fargo CMBS    
Effective rate           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           Short Hills Portfolio    
Lender           Wells Fargo CMBS    
Effective rate           4.15%    
Principal balance outstanding           $ 124,500,000 124,500,000  
Loan maturity date           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           4.48%    
Principal balance outstanding           $ 41,000,000 41,000,000  
Loan maturity date           Aug. 05, 2027    
Secured Debt [Member] | Port Imperial South 11 [Member]                
Debt Instrument [Line Items]                
Property Name           Port Imperial South 11    
Lender           The Northwestern Mutual Life Insurance Co.    
Effective rate           4.52%    
Principal balance outstanding           $ 100,000,000 46,113,000  
Loan maturity date           Jan. 10, 2029    
Maximum borrowing capacity $ 100,000,000              
Unamortized deferred financing costs 300,000              
Secured Debt [Member] | Port Imperial South 11 [Member] | Construction Loan [Member]                
Debt Instrument [Line Items]                
Payment for borrowings $ 70,100,000              
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           4.85%    
Principal balance outstanding           $ 32,600,000 $ 32,600,000  
Loan maturity date           Dec. 01, 2029