Mortgages Payable (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage payable (in US Dollars)	$ 1,054,046,000	$ 1,065,916,000
Rego Park 1 Shopping [Member] | Mortgages [Member] | Secured Debt [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2015-03
Interest rate	0.40%
Mortgage payable (in US Dollars)	78,246,000	78,246,000
Percentage of cash mortgage collateralized	100.00%
Rego Park 2 Property [Member] | Mortgages [Member] | Secured Debt [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2018-11
Interest rate	2.03%
Mortgage payable (in US Dollars)	270,203,000	272,245,000
Basis spread over LIBOR	1.85%
Lexington Avenue Property [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage payable (in US Dollars)	320,000,000
Mortgage Loan Converted To Recourse (in US Dollars)	75,000,000
Lexington Avenue Property [Member] | Mortgages [Member] | Secured Debt [Member] | Office Space [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2014-02
Interest rate	5.33%
Mortgage payable (in US Dollars)	317,597,000	327,425,000
Lexington Avenue Property [Member] | Mortgages [Member] | Secured Debt [Member] | Retail Site [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2015-07
Interest rate	4.93%
Mortgage payable (in US Dollars)	320,000,000	320,000,000
Lexington Avenue Property [Member] | Mortgages [Member] | Secured Debt [Member] | Retail Site [Member] | Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loan Converted To Recourse (in US Dollars)	75,000,000
Paramus Property [Member] | Mortgages [Member] | Secured Debt [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2018-10
Interest rate	2.90%
Mortgage payable (in US Dollars)	$ 68,000,000	$ 68,000,000