Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)				9 Months Ended
	Mar. 29, 2018 USD ($)	Mar. 01, 2018 USD ($)	Jan. 08, 2018 USD ($)	Sep. 30, 2018 USD ($) property item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Principal balance outstanding				$ 2,807,718,000		$ 2,809,568,000
Borrowings from revolving credit facility				428,000,000	$ 428,000,000
Payment for borrowings				381,000,000	$ 714,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Principal balance outstanding				1,374,792,000		1,426,111,000
Unamortized deferred financing costs				(7,409,000)		(7,976,000)
Total mortgages, loans payable and other obligations, net				$ 1,367,383,000		1,418,135,000
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				6.84%
Principal balance outstanding						209,257,000
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
Spread over LIBOR				1.87%
Principal balance outstanding				$ 25,517,000		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
Spread over LIBOR				2.35%
Principal balance outstanding				$ 41,768,000		32,491,000
Loan maturity date				May 20, 2019
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] | Portside 5/6 [Member]
Debt Instrument [Line Items]
Property Name				Portside 5/6
Lender				Citizens Bank
LIBOR				LIBOR+2.50
Spread over LIBOR				2.50%
Principal balance outstanding				$ 69,787,000		45,778,000
Loan maturity date				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				Port Imperial 4/5 Hotel
Lender				Fifth Third Bank & Santander
LIBOR				LIBOR+4.50
Spread over LIBOR				4.50%
Principal balance outstanding				$ 68,928,000		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] | Port Imperial South 11 [Member]
Debt Instrument [Line Items]
Property Name				Port Imperial South 11
Lender				JPMorgan Chase
LIBOR				LIBOR+2.35
Spread over LIBOR				2.35%
Principal balance outstanding				$ 67,427,000		46,113,000
Loan maturity date				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				Worcester
Lender				Citizens Bank
LIBOR				LIBOR+2.50
Spread over LIBOR				2.50%
Principal balance outstanding				$ 54,743,000		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				$ 168,774,000		169,987,000
Loan maturity date				Feb. 01, 2021
Adjustment for unamortized debt discount				$ 3,800,000
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] | Marbella [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 Street [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 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