Mortgages, Loans Payable And Other Obligations (Summary Of Mortgages, Loans Payable And Other Obligations) (Details)				9 Months Ended
	Jul. 29, 2019 USD ($)	Jun. 28, 2019 USD ($)	Jan. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Jan. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Principal balance outstanding				$ 3,151,561,000			$ 2,792,651,000
Borrowings from revolving credit facility				489,000,000	$ 428,000,000
Payment for borrowings				$ 398,000,000	$ 381,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Effective rate				3.15%
Principal balance outstanding				$ 2,106,550,000			1,440,396,000
Unamortized deferred financing costs				13,917,000			8,998,000
Total mortgages, loans payable and other obligations, net				$ 2,092,633,000			1,431,398,000
Secured Debt [Member] | Park Square Apartments [Member]
Debt Instrument [Line Items]
Property Name				Park Square (b)
Lender				Wells Fargo Bank N.A.
Effective rate				1.87%
Principal balance outstanding							25,167,000
Secured Debt [Member] | Worcester [Member]
Debt Instrument [Line Items]
Property Name				Worcester (c)
Lender				Citizens Bank
Effective rate				2.50%
Principal balance outstanding				$ 57,883,000			56,892,000
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 (d)
Lender				The Northwestern Mutual Life Insurance Co.
Effective rate				3.15%
Principal balance outstanding				$ 167,157,000			168,370,000
Loan maturity date				Feb. 01, 2021
Secured Debt [Member] | Monaco [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 2,100,000
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				$ 3,951,000			4,000,000
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 (e)
Lender				Fifth Third Bank & Santander
Effective rate				4.50%
Principal balance outstanding				$ 56,636,000			73,350,000
Loan maturity date				Apr. 09, 2022
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%
Payment for borrowings		$ 30,000,000
Secured Debt [Member] | Chase III Project [Member]
Debt Instrument [Line Items]
Property Name				Chase III (f)
Lender				Fifth Third Bank
Effective rate				2.50%
Principal balance outstanding				$ 13,837,000
Loan maturity date				May 16, 2022
Secured Debt [Member] | Chase III Project [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 62,000,000
Number of extension options | item				1
Loan extension period				18 months
Extension fee				0.25%
Secured Debt [Member] | Port Imperial South 9 [Member]
Debt Instrument [Line Items]
Property Name				Port Imperial South 9 (g)
Lender				Bank of New York Mellon
Effective rate				2.13%
Principal balance outstanding				$ 4,238,000
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%
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 (h)
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] | 250 Johnson Road [Member]
Debt Instrument [Line Items]
Property Name				250 Johnson (i)
Lender				Nationwide Life Insurance Company
Effective rate				3.74%
Principal balance outstanding				$ 43,000,000			41,769,000
Loan maturity date				Aug. 01, 2024
Secured Debt [Member] | 250 Johnson Road [Member] | Construction Loan [Member]
Debt Instrument [Line Items]
Loan maturity date				Aug. 01, 2024
Payment for borrowings	$ 43,000,000
Secured Debt [Member] | Liberty Towers [Member]
Debt Instrument [Line Items]
Property Name				Liberty Towers
Lender				American General Life Insurance Company
Effective rate				3.37%
Principal balance outstanding				$ 232,000,000
Loan maturity date				Oct. 01, 2024
Secured Debt [Member] | The Chase At Overlook Ridge [Member]
Debt Instrument [Line Items]
Property Name				The Chase at Overlook Ridge (j)
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			97,000,000
Loan maturity date				Mar. 10, 2026
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			131,000,000
Loan maturity date				Aug. 10, 2026
Secured Debt [Member] | Marbella II [Member]
Debt Instrument [Line Items]
Property Name				Marbella II (k)
Lender				New York Life Insurance Company
Effective rate				4.29%
Principal balance outstanding				$ 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] | Short Hills Portfolio [Member]
Debt Instrument [Line Items]
Property Name				Short Hills Portfolio (l)
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			100,000,000
Loan maturity date				Jan. 10, 2029
Secured Debt [Member] | Soho Lofts Apartments [Member]
Debt Instrument [Line Items]
Property Name				Soho Lofts
Lender				New York Community Bank
Effective rate				3.77%
Principal balance outstanding				$ 160,000,000
Loan maturity date				Jul. 01, 2029
Secured Debt [Member] | Riverwatch Commons [Member]
Debt Instrument [Line Items]
Property Name				Riverwatch Commons (m)
Lender				New York Community Bank
Effective rate				3.79%
Principal balance outstanding				$ 30,000,000
Loan maturity date				Jul. 01, 2029
Effective percentage after five years				2.75%
First period of term period				5 years
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
Loan maturity date				Sep. 01, 2029
Secured Debt [Member] | Port Imperial South 4/5 Garage [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
Unconsolidated Joint Venture Multi-Family Properties [Member] | Marbella II [Member]
Debt Instrument [Line Items]
Percentage of additional interest acquired			50.00%
Mortgage loan			$ 117,000,000			$ 74,700,000