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