Mortgages Payable - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2023	Dec. 31, 2022
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Notes payable	$ 1,092,125	$ 1,091,051
Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgage payable gross	1,096,544	1,096,544
Deferred debt issuance costs, net of accumulated amortization of $17,246 and $16,071, respectively	(4,419)	(5,493)
Notes payable	1,092,125	1,091,051
Deferred debt issuance costs, accumulated amortization	$ 17,246	16,071
Mortgages \| 731 Lexington Avenue \| Prime Rate
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate	8.50%
Mortgages \| 731 Lexington Avenue \| Office Space
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate (in percentage)	6.00%
Mortgage payable gross	$ 500,000	500,000
Mortgages \| 731 Lexington Avenue \| Office Space \| Prime Rate
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate	6.00%
Mortgages \| 731 Lexington Avenue \| Retail Space
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate (in percentage)	1.76%
Mortgage payable gross	$ 300,000	300,000
Mortgages \| 731 Lexington Avenue \| Retail Space \| Secured Overnight Financing Rate (SOFR)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread over SOFR	1.51%
Mortgages \| Rego Park II \| Retail Space
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate (in percentage)	5.60%
Mortgage payable gross	$ 202,544	202,544
Mortgages \| Rego Park II \| Retail Space \| Secured Overnight Financing Rate (SOFR)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread over SOFR	1.45%
Mortgages \| Rego Park II \| Retail Space \| Secured Overnight Financing Rate (SOFR) \| Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest cap rate	4.15%
Mortgages \| Alexander Apartment Tower \| Apartment Building
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Interest rate (in percentage)	2.63%
Mortgage payable gross	$ 94,000	$ 94,000

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- References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 948
-SubTopic 310
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29)
-Publisher FASB
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Amount of accumulated amortization of debt issuance costs.

+ References

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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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

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- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ 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://fasb.org/us-gaap/role/ref/legacyRef
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