Investments in Partially Owned Entities (Alexander's Inc.) (Details)			12 Months Ended
	Jan. 15, 2015 USD ($) Properties	Dec. 22, 2014 USD ($)	Dec. 31, 2015 USD ($) $ / shares $ / ft² shares	Dec. 31, 2014 USD ($) Properties	Dec. 31, 2013 USD ($)
Managment And Development Agreement [Abstract]
Cash Considerations Transferred During Spin Off			$ 225,000,000	$ 0	$ 0
Urban Edge Properties [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Closing share price (in dollars per share) | $ / shares			$ 23.45
Equity Method Investment Market Value			$ 134,068,000
Excess of investee's fair value over carrying amount			$ 108,717,000
Urban Edge Properties [Member] | Discontinued Operations [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	5.40%
Managment And Development Agreement [Abstract]
Cash Considerations Transferred During Spin Off	$ 225,000,000
Retail Segment Strip Shopping Centers [Member] | Discontinued Operations [Member]
Managment And Development Agreement [Abstract]
Number Of Real Estate Properties | Properties				22
Retail Segment Strip Shopping Centers [Member] | Urban Edge Properties [Member] | Discontinued Operations [Member]
Managment And Development Agreement [Abstract]
Number Of Real Estate Properties | Properties	79
Retail Segment Regional Malls [Member] | Urban Edge Properties [Member] | Discontinued Operations [Member]
Managment And Development Agreement [Abstract]
Number Of Real Estate Properties | Properties	3
Warehouse [Member] | Urban Edge Properties [Member] | Discontinued Operations [Member]
Managment And Development Agreement [Abstract]
Number Of Real Estate Properties | Properties	1
Alexanders Inc [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Ownership common shares, investee (in shares) | shares			1,654,068
Equity method ownership percentage			32.40%
Closing share price (in dollars per share) | $ / shares			$ 384.11
Equity Method Investment Market Value			$ 635,345,000
Excess of investee's fair value over carrying amount			501,777,000
Excess of investee's carrying amount over equity in net assets			$ 40,340,000
Managment And Development Agreement [Abstract]
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) $289,000, 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 [Abstract]
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 Agreements [Abstract]
Fee Income recognized by equity method investees for building maintenance services			$ 2,221,000	$ 2,318,000	$ 2,036,000
Other Financial Data For Equity Method Investments [Abstract]
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.
Alexanders Inc [Member] | Rego Park 2 Property [Member]
Managment And Development Agreement [Abstract]
Property Management Fee Agreement Percentage Of Income			2.00%
Alexanders Inc [Member] | Lexington Avenue 731 [Member]
Managment And Development Agreement [Abstract]
Property Management Fee Agreement Price Per Square Foot | $ / ft²			0.50
Alexanders Inc [Member] | Common Area 731 Lexington Avenue [Member]
Managment And Development Agreement [Abstract]
Management and Development fee income			$ 289,000
Property Management Fee Agreement Escalation Percentage Of Income			3.00%