Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)	1 Months Ended		9 Months Ended
	Apr. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Oct. 27, 2021 USD ($)	Jan. 31, 2020 USD ($)
Debt Instrument [Line Items]
Principal balance outstanding		$ 2,801,797,000	$ 2,374,947,000
Repayment of revolving credit facility and money market loans			$ 37,000,000	$ 364,000,000
RXR - Short Hills [Member]
Debt Instrument [Line Items]
Loan maturity date			Apr. 01, 2027
Port Imperial 4/5 Garage Development [Member]
Debt Instrument [Line Items]
Loan maturity date			Dec. 01, 2029
Deferred interest			$ 800,000
Subsequent Event [Member] | Short Hills Residential [Member]
Debt Instrument [Line Items]
Mortgage loan					$ 75,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Principal balance outstanding		2,218,862,000	2,212,663,000
Unamortized deferred financing costs		(14,718,000)	(11,716,000)
Total mortgages, loans payable and other obligations, net		2,204,144,000	$ 2,200,947,000
Secured Debt [Member] | Port Imperial South 4/5 Retail [Member]
Debt Instrument [Line Items]
Property Name			Port Imperial South 4/5 Retail (m)
Lender			American General Life & A/G PC
Effective rate			4.56%
Principal balance outstanding		3,866,000	$ 3,813,000
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	$ 89,000,000
Loan maturity date			Apr. 01, 2023
Loan extension period			6 months
Guaranteed amount			$ 14,500,000
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	$ 79,609,000
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
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			Upton-Short Hills (d)
Lender			People's United Bank
Effective rate			2.15%
Principal balance outstanding		42,459,000	$ 57,171,000
Loan maturity date			Mar. 26, 2023
Secured Debt [Member] | Short Hills Residential [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
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]
Maximum borrowing capacity						$ 265,000,000
Additional borrowing capacity						$ 33,000,000
Secured Debt [Member] | The Charlotte [Member]
Debt Instrument [Line Items]
Property Name			Haus 25 (f)
Lender			QuadReal Finance
Effective rate			2.70%
Principal balance outstanding		161,544,000	$ 237,288,000
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			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			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] | RXR - Short Hills [Member]
Debt Instrument [Line Items]
Property Name			RXR - Short Hills (h)
Lender			Wells Fargo CMBS
Effective rate			4.15%
Principal balance outstanding		124,500,000
Closing costs to defease loan	$ 22,600,000
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			BLVD 475 N/S (i)
Lender			The Northwestern Mutual Life Insurance Co.
Effective rate			2.91%
Principal balance outstanding		165,000,000	$ 165,000,000
Loan maturity date			Nov. 10, 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	$ 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 (j)
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 (k)
Lender			American General Life & A/G PC
Effective rate			4.85%
Principal balance outstanding		33,138,000	$ 32,784,000
Secured Debt [Member] | Emery At Overlook Ridge [Member]
Debt Instrument [Line Items]
Property Name			Emery at Overlook Ridge (l)
Lender			New York Community Bank
Effective rate			3.21%
Principal balance outstanding		72,000,000	$ 72,000,000
Loan maturity date			Jan. 01, 2031
Mortgage loan		72,000,000
Gain (loss) on sale of property		$ 10,400,000