Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)	1 Months Ended				3 Months Ended
	Apr. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Nov. 30, 2020	Jan. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) item	Mar. 31, 2020 USD ($)
Debt Instrument [Line Items]
Principal balance outstanding		$ 2,801,797,000			$ 2,821,964,000
Repayment of revolving credit facility and money market loans					$ 33,000,000	$ 121,000,000
Port Imperial 4/5 Garage Development [Member]
Debt Instrument [Line Items]
Loan maturity date					Dec. 01, 2029
Subsequent Event [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Debt Instrument [Line Items]
Repayment of debt	$ 124,500,000
Subsequent Event [Member] | Short Hills Portfolio [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Debt Instrument [Line Items]
Repayment of debt	$ 22,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Principal balance outstanding		2,218,862,000			$ 2,262,880,000
Unamortized deferred financing costs		(14,718,000)			(13,861,000)
Total mortgages, loans payable and other obligations, net		2,204,144,000			$ 2,249,019,000
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		3,866,000			$ 3,848,000
Loan maturity date					Dec. 01, 2021
Secured Debt [Member] | Port Imperial 4/5 Hotel [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial 4/5 Hotel (b)
Lender					Fifth Third Bank
Effective rate					3.40%
Principal balance outstanding		94,000,000			$ 94,000,000
Loan maturity date					Apr. 09, 2022
Guaranteed amount					$ 19,500,000
Secured Debt [Member] | Port Imperial South 9 [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial South 9 (c)
Lender					Bank of New York Mellon
Effective rate					2.13%
Principal balance outstanding		46,357,000			$ 60,253,000
Loan maturity date					Dec. 19, 2022
Secured Debt [Member] | Port Imperial South 9 [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 92,000,000
Number of extension options | item					1
Loan extension period					1 year
Extension fee					0.15%
Percent of principal loan outstanding					10.00%
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] | Short Hills Residential [Member]
Debt Instrument [Line Items]
Property Name					Short Hills Residential (d)
Lender					People's United Bank
Effective rate					2.15%
Principal balance outstanding		42,459,000			$ 50,395,000
Loan maturity date					Mar. 26, 2023
Secured Debt [Member] | Short Hills Residential [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Number of extension options | item					1
Loan extension period					18 months
Percent of principal loan outstanding					15.00%
Secured Debt [Member] | 250 Johnson Road [Member]
Debt Instrument [Line Items]
Property Name					250 Johnson
Lender					Nationwide Life Insurance Company
Effective rate					3.74%
Principal balance outstanding		43,000,000			$ 43,000,000
Loan maturity date					Aug. 01, 2024
Secured Debt [Member] | Liberty Towers [Member]
Debt Instrument [Line Items]
Property Name					Liberty Towers (e)
Lender					American General Life Insurance Company
Effective rate					3.37%
Principal balance outstanding		265,000,000			$ 265,000,000
Loan maturity date					Oct. 01, 2024
Secured Debt [Member] | Liberty Towers [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Additional borrowing capacity				$ 33,000,000
Repayment of revolving credit facility and money market loans				$ 265,000,000
Secured Debt [Member] | The Charlotte [Member]
Debt Instrument [Line Items]
Property Name					The Charlotte (f)
Lender					QuadReal Finance
Effective rate					2.70%
Principal balance outstanding		161,544,000			$ 183,863,000
Loan maturity date					Dec. 01, 2024
Secured Debt [Member] | The Charlotte [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Spread over LIBOR					2.00%
Maximum borrowing capacity					$ 300,000,000
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					Portside 5/6 (g)
Lender					New York Life Insurance Company
Effective rate					4.56%
Principal balance outstanding		97,000,000			$ 97,000,000
Loan maturity date					Mar. 10, 2026
Secured Debt [Member] | Marbella [Member]
Debt Instrument [Line Items]
Property Name					Marbella (BLVD 425)
Lender					New York Life Insurance Company
Effective rate					4.17%
Principal balance outstanding		131,000,000			$ 131,000,000
Loan maturity date					Aug. 10, 2026
Secured Debt [Member] | Marbella II [Member]
Debt Instrument [Line Items]
Property Name					Marbella II (BLVD 401)
Lender					New York Life Insurance Company
Effective rate					4.29%
Principal balance outstanding		117,000,000			$ 117,000,000
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] | Worcester [Member]
Debt Instrument [Line Items]
Property Name					Worcester
Lender					MUFG Union Bank
Effective rate					1.84%
Principal balance outstanding		63,000,000			$ 63,000,000
Loan maturity date					Dec. 10, 2026
Secured Debt [Member] | Short Hills Portfolio [Member]
Debt Instrument [Line Items]
Property Name					Short Hills Portfolio (h)
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] | Short Hills Portfolio [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 64,000,000
Extension fee					0.30%
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] | Monaco (BLVD 495 N/S) [Member]
Debt Instrument [Line Items]
Property Name					Monaco (BLVD 495 N/S) (l)
Lender					The Northwestern Mutual Life Insurance Co.
Effective rate					2.91%
Principal balance outstanding		$ 165,000,000			$ 165,000,000
Loan maturity date		Nov. 01, 2027	Feb. 01, 2021		Nov. 10, 2027
Guaranteed amount					$ 12,000,000
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			$ 100,000,000
Loan maturity date					Jan. 10, 2029
Percent of principal loan outstanding					10.00%
Secured Debt [Member] | Soho Lofts [Member]
Debt Instrument [Line Items]
Property Name					Soho Lofts (i)
Lender					New York Community Bank
Effective rate					3.77%
Principal balance outstanding		160,000,000			$ 160,000,000
Loan maturity date					Jul. 01, 2029
Secured Debt [Member] | Riverwatch Commons [Member]
Debt Instrument [Line Items]
Effective percentage after five years					2.75%
Secured Debt [Member] | 111 River St. [Member]
Debt Instrument [Line Items]
Property Name					111 River St.
Lender					Athene Annuity and Life Company
Effective rate					3.90%
Principal balance outstanding		150,000,000			$ 150,000,000
Loan maturity date					Sep. 01, 2029
Secured Debt [Member] | Port Imperial 4/5 Garage Development [Member]
Debt Instrument [Line Items]
Property Name					Port Imperial South 4/5 Garage (j)
Lender					American General Life & A/G PC
Effective rate					4.85%
Principal balance outstanding		33,138,000			$ 33,023,000
Deferred interest					$ 800,000
Deferred interest and payment end date					Dec. 31, 2020
Secured Debt [Member] | Emery At Overlook Ridge [Member]
Debt Instrument [Line Items]
Property Name					Emery at Overlook Ridge (k)
Lender					New York Community Bank
Effective rate					3.21%
Principal balance outstanding		$ 72,000,000			$ 72,000,000
Loan maturity date		Jan. 01, 2031			Jan. 01, 2031
Mortgage loan		$ 72,000,000
Gain (loss) on sale of property		$ 10,400,000