Mortgages Payable (Outstanding mortgages payable) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate
Notes Payable (in US dollars)	$ 1,051,973	$ 1,052,359
Rego Park I
Mortgage Loans on Real Estate
Percentage of cash mortgage collateralized	100.00%
Mortgages \| Secured loan
Mortgage Loans on Real Estate
Mortgage payable gross	$ 1,055,246	1,056,147
Deferred debt issuance costs, net of accumulated amortization of $7,348 and $6,824 respectively	(3,273)	(3,788)
Notes Payable (in US dollars)	1,051,973	1,052,359
Deferred debt issuance costs, accumulated amortization (in US dollars)	$ 7,348	6,824
Mortgages \| Secured loan \| Rego Park I \| Retail space
Mortgage Loans on Real Estate
Maturity date	Mar. 10, 2018
Interest rate (in percentage)	0.35%
Percentage of cash mortgage collateralized	100.00%
Mortgage payable gross	$ 78,246	78,246
Mortgage loan extension date	2016-03
Duration Of Mortgage Loan Extension	2 years
Mortgages \| Secured loan \| Rego Park II \| Retail space
Mortgage Loans on Real Estate
Maturity date	Nov. 30, 2018
Interest rate (in percentage)	2.83%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Mortgage payable gross	$ 259,000	259,901
Mortgages \| Secured loan \| Rego Park II \| Retail space \| LIBOR
Mortgage Loans on Real Estate
Basis spread over LIBOR (in percentage)	1.85%
Mortgages \| Secured loan \| 731 Lexington Avenue \| Office space
Mortgage Loans on Real Estate
Maturity date	Mar. 11, 2021
Interest rate (in percentage)	1.86%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Mortgage payable gross	$ 300,000	300,000
Mortgages \| Secured loan \| 731 Lexington Avenue \| Office space \| LIBOR
Mortgage Loans on Real Estate
Basis spread over LIBOR (in percentage)	0.95%
Mortgages \| Secured loan \| 731 Lexington Avenue \| Retail space
Mortgage Loans on Real Estate
Maturity date	Aug. 31, 2022
Interest rate (in percentage)	2.23%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Mortgage payable gross	$ 350,000	350,000
Mortgages \| Secured loan \| 731 Lexington Avenue \| Retail space \| LIBOR
Mortgage Loans on Real Estate
Basis spread over LIBOR (in percentage)	1.40%
Mortgages \| Secured loan \| Paramus
Mortgage Loans on Real Estate
Maturity date	Oct. 05, 2018
Interest rate (in percentage)	2.90%
Mortgage payable gross	$ 68,000	$ 68,000

The term of the extension, in years.

No definition available.

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- Definition

Mortgage loan extension date in the YYYY-MM format.

+ References

No definition available.

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- References

No definition available.

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- Definition

Percentage of cash mortgage collateralized

+ References

No definition available.

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- Definition

Amount of accumulated amortization of debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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