Investments in Partially Owned Entities (Alexander's Inc.) (Details) - Alexanders Inc	1 Months Ended	12 Months Ended
	Dec. 22, 2014 USD ($)	Dec. 31, 2016 USD ($) $ / shares $ / ft² shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Equity Method Investments And Income From Equity Method Investments
Ownership common shares, investee (in shares) | shares		1,654,068
Equity method ownership percentage		32.40%
Closing share price (in dollars per share) | $ / shares		$ 426.87
Equity Method Investment Market Value		$ 706,072,000
Excess of investee's fair value over carrying amount		576,748,000
Excess of investee's carrying amount over equity in net assets		39,723,000
Amount Alexander owed		$ 1,070,000	$ 8,551,000
Managment And Development Agreement
Property Management 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) $297,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Management and Development 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%
Asset Sale Commission Threshold		$ 50,000,000
Percentage Commissions On Sale Of Assets Under Fifty Million		3.00%
Percentage Commissions On Sale Of Assets Over Fifty Million		1.00%
Leasing Services Fee And Commission On Asset Sales Annual Installment Eliminated	$ 4,000,000
Repayment of Alexander's Inc. leasing commissions	$ 40,353,000
Other Financial Data For Equity Method Investments
Commission On Asset Sales Description		commission upon the sale of any of Alexander’s assets equal to 3% of gross proceeds, as defined, for asset sales less than $50,000,000, and 1% of gross proceeds, as defined, for asset sales of $50,000,000 or more.
Building Maintenance Service ("BMS")
Other Agreements
Fee Income recognized by equity method investees for building maintenance services		$ 2,583,000	$ 2,221,000	$ 2,318,000
Rego Park 2 Property
Managment And Development Agreement
Property Management Fee Agreement Percentage Of Income		2.00%
Lexington Avenue 731
Managment And Development Agreement
Property Management Fee Agreement Price Per Square Foot | $ / ft²		0.5
Common Area 731 Lexington Avenue
Managment And Development Agreement
Management and Development fee income		$ 297,000
Property Management Fee Agreement Escalation Percentage Of Income		3.00%