Investments in Partially Owned Entities (Alexander's Inc.) (Details)		1 Months Ended	12 Months Ended
	Jun. 02, 2017 USD ($) myExtension	May 31, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares $ / ft² shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Equity Method Investments And Income From Equity Method Investments
Mortgages payable, net			$ 8,137,139,000	$ 8,113,248,000
Alexanders Inc
Equity Method Investments And Income From Equity Method Investments
Investment owned, balance, shares | shares			1,654,068
Equity method ownership percentage			32.40%
Amount Alexander owed			$ 2,490,000	1,070,000
Closing share price (in dollars per share) | $ / shares			$ 395.85
Equity method investment, quoted market value			$ 654,763,000
Excess of investee's fair value over carrying amount			528,363,000
Excess of investee's carrying amount over equity in net assets			$ 39,367,000
Debt instrument, interest rate, effective percentage			2.61%
Managment Agreement
Property management fee, percent fee, description			We receive an annual fee for managing Alexander’s and all of its properties equal to the sum of (i) $2,800,000, (ii) 2% of the gross revenue from the Rego Park II Shopping Center, (iii) $0.50 per square foot of the tenant-occupied office and retail space at 731 Lexington Avenue, and (iv) $306,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Management fee income			$ 2,800,000
Development fee percent			6.00%
Leasing Agreement
Leasing services fee description			leasing services for a fee of 3% of rent for the first ten years of a lease term, 2% of rent for the eleventh through twentieth year of a lease term and 1% of rent for the twenty-first through thirtieth year of a lease term, subject to the payment of rents by Alexander’s tenants.  In the event third-party real estate brokers are used, our fee increases by 1% and we are responsible for the fees to the third-parties.
Lease fee percentage of rent one to ten years			3.00%
Lease fee percentage of rent eleven to twenty years			2.00%
Lease fee percentage of rent twenty first to thirty years			1.00%
Percentage increase lease fee if broker used			1.00%
Percentage commissions on sale of assets under fifty million			3.00%
Asset sale commission threshold			$ 50,000,000
Percentage commissions on sale of assets over fifty million			1.00%
Alexanders Inc | Building Maintenance Service ("BMS")
Other Agreements
Fee Income recognized by equity method investees for building maintenance services			$ 2,678,000	$ 2,583,000	$ 2,221,000
Alexanders Inc | Lexington Avenue 731
Managment Agreement
Property management fee agreement price per square foot | $ / ft²			0.50
Alexanders Inc | Rego Park 2 Property
Managment Agreement
Property management fee agreement percentage of income			2.00%
Alexanders Inc | Common Area 731 Lexington Avenue
Managment Agreement
Management fee income			$ 306,000
Property management fee agreement escalation percentage of income			3.00%
Mortgage Loan Maturing in June 2020 | Office | Alexanders Inc | Lexington Avenue 731
Equity Method Investments And Income From Equity Method Investments
Mortgages payable, net	$ 500,000,000
Debt instrument, interest rate, effective percentage			2.38%
Number of loan extensions available | myExtension	4
Length of extension available	1 year
Mortgage Loan Maturing in March 2021 | Office | Alexanders Inc | Lexington Avenue 731
Equity Method Investments And Income From Equity Method Investments
Mortgages payable, net		$ 300,000,000
LIBOR | Mortgage Loan Maturing in June 2020 | Office | Alexanders Inc | Lexington Avenue 731
Equity Method Investments And Income From Equity Method Investments
Spread Over LIBOR (in percentage)	0.90%
LIBOR | Mortgage Loan Maturing in March 2021 | Office | Alexanders Inc | Lexington Avenue 731
Equity Method Investments And Income From Equity Method Investments
Spread Over LIBOR (in percentage)		0.95%
Interest Rate Cap | Office | Alexanders Inc | Lexington Avenue 731
Equity Method Investments And Income From Equity Method Investments
Derivative liability, notional amount	$ 500,000,000
Derivative, cap interest rate	6.00%