Investments in Partially Owned Entities (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended									1 Months Ended	12 Months Ended						2 Months Ended	12 Months Ended						12 Months Ended								12 Months Ended								12 Months Ended																12 Months Ended							3 Months Ended	12 Months Ended										12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Aug. 31, 2010 Toys R Us [Member]	Dec. 31, 2011 Toys R Us [Member]	Oct. 29, 2011 Toys R Us [Member]	Dec. 31, 2010 Toys R Us [Member]	Oct. 30, 2010 Toys R Us [Member]	Dec. 31, 2009 Toys R Us [Member]	Oct. 31, 2009 Toys R Us [Member]	Dec. 31, 2011 Alexanders Inc [Member]	Dec. 31, 2010 Alexanders Inc [Member]	Dec. 31, 2009 Alexanders Inc [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Dec. 31, 2010 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Dec. 31, 2009 Lexington Realty Trust [Member]	Sep. 30, 2009 Lexington Realty Trust [Member]	Sep. 30, 2010 LNR [Member]	Dec. 31, 2011 LNR [Member]	Sep. 30, 2011 LNR [Member]	Dec. 31, 2010 LNR [Member]	Dec. 31, 2009 LNR [Member]	Dec. 31, 2011 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Dec. 31, 2010 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Dec. 31, 2011 India real estate ventures [Member]	Dec. 31, 2010 India real estate ventures [Member]	Dec. 31, 2009 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member] Range minimum [Member]	Dec. 31, 2011 India real estate ventures [Member] Range maximum [Member]	Dec. 31, 2011 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Mar. 16, 2011 Partially owned office buildings [Member] Vornado Realty Trust [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2009 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2009 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range minimum [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range maximum [Member]	Dec. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2009 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Warner Building and 1101 17th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Warner Building and 1101 17th Street [Member]	Dec. 31, 2009 Partially owned office buildings [Member] Warner Building and 1101 17th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Dec. 31, 2009 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 666 Fifth Avenue [Member] sqft	Dec. 16, 2011 Partially owned office buildings [Member] 666 Fifth Avenue [Member]	Dec. 16, 2011 Partially owned office buildings [Member] 666 Fifth Avenue [Member] Vornado Realty Trust [Member]	Dec. 16, 2011 Partially owned office buildings [Member] 666 Fifth Avenue [Member] Vornado Realty Trust and Kushner Companies [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 825 Seventh Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member]	Dec. 31, 2009 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member]	Mar. 16, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member] Vornado Realty Trust [Member]	Mar. 16, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member] SL Green [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member] Vornado Realty Trust And SL Green [Member]	Mar. 16, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member] Vornado Realty Trust And SL Green [Member]	Mar. 16, 2011 Other equity method investments [Member] sqft	Dec. 31, 2010 Other equity method investments [Member]	Dec. 31, 2009 Other equity method investments [Member]	Dec. 31, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2010 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2009 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2011 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2010 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2009 Other equity method investments [Member] Independence Plaza [Member]	Jun. 17, 2011 Other equity method investments [Member] Independence Plaza [Member]	Jun. 17, 2011 Other equity method investments [Member] Independence Plaza [Member] Vornado Realty Trust [Member]	Dec. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2010 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2009 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2010 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2009 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2011 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2010 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2009 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2011 Other equity method investments [Member] Green Courte Real Estate Partners Llc [Member]	Dec. 31, 2011 Other equity method investments [Member] Wells Kinzie Garage [Member]	Dec. 31, 2011 Other equity method investments [Member] Orleans Hubbard Garage [Member]
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	$ 153,861,000,000	$ 165,183,000,000				$ 13,221,000,000		$ 12,810,000,000			$ 1,771,000,000	$ 1,679,000,000				$ 3,164,000,000		$ 3,385,000,000			$ 143,266,000,000		$ 128,536,000,000			$ 127,000,000,000	$ 142,000,000,000
Liabilities	147,854,000,000	160,203,000,000				11,530,000,000		11,317,000,000			1,408,000,000	1,335,000,000				1,888,000,000		2,115,000,000			142,720,000,000		127,809,000,000			127,000,000,000	142,000,000,000
Noncontrolling interests	132,000,000	124,000,000									4,000,000	3,000,000				59,000,000		71,000,000			37,000,000		55,000,000
Equity	5,875,000,000	4,856,000,000				1,691,000,000		1,493,000,000			359,000,000	341,000,000				1,217,000,000		1,199,000,000			509,000,000		672,000,000
Total revenue	15,390,000,000	15,030,000,000	14,321,000,000			13,956,000,000		13,749,000,000		13,172,000,000	254,000,000	242,000,000	224,000,000			335,000,000		330,000,000		375,000,000	23,000,000		208,000,000
Net (loss) income attributable to investee	199,000,000	63,000,000	103,000,000			121,000,000		189,000,000		216,000,000	79,000,000	67,000,000	132,000,000			(81,000,000)		(90,000,000)		(178,000,000)	8,000,000		224,000,000
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage					32.70%						32.40%				12.00%		12.80%		15.20%			26.20%									4.00%	36.50%	25.00%		49.50%	50.00%						43.70%	50.40%	30.30%			55.00%						49.50%				25.00%	20.00%	50.00%							99.00%				8.30%			51.00%					50.00%			50.00%						8.30%	50.00%	50.00%
Carrying amount of investments in partially owned entities	1,740,459,000	1,375,006,000	1,209,285,000		506,809,000		447,334,000				189,775,000	186,811,000			57,402,000		57,270,000					174,408,000		132,973,000				80,499,000	127,193,000							58,529,000	58,963,000		53,333,000	52,689,000				47,568,000	0		23,122,000	37,741,000		61,898,000	36,487,000									184,516,000	0									59,801,000	59,326,000		48,511,000	0				46,691,000	46,147,000		7,536,000	6,251,000		140,061,000	125,821,000
Equity in net income, before income taxes					38,460,000		16,401,000		58,416,000
Income tax expense					1,132,000		45,418,000		13,185,000													12,380,000
Equity in net income before stock appreciation rights											25,013,000	20,059,000	17,991,000
Equity in net income					39,592,000		61,819,000		71,601,000		25,013,000	20,059,000	42,764,000
Interest and other income					8,948,000		9,805,000		6,753,000
Gain on stock issuance															9,760,000		13,710,000
Non-cash impairment charges														107,882,000																																																						11,481,000	17,820,000
Net gain on disposition of wholly owned and partially owned assets	15,134,000	81,432,000	5,641,000
Straight line rent reserves and write off of tenant improvements																																															9,022,000
Tax Adjustments, Settlements, and Unusual Provisions																						8,977,000
Net gains from asset sales																						6,020,000
Impairment loss																			19,121,000									13,794,000																																												14,515,000														3,305,000
Gain on sale of partial interest																																																																																				12,525,000
Other Financial Data For Equity Method Investments [Abstract]
Toy's fourth quarter fiscal year net income					more than 80%
Subsidiary or Equity Method Investee, Price-Per-Share											$ 370.03				$ 7.49
Investment Advisory Fees				9,600,000
Ownership common shares, investee											1,654,068				18,468,969
Equity Method Investment Market Value											612,055,000				138,333,000
Excess of investee's fair value over carrying amount											422,280,000				80,931,000
Excess of investee's carrying amount over equity in net assets											59,010,000				(49,200,000)
Development Fee Percent											6.00%
Development Fee Minimum Guaranteed Amount											750,000
Development fee income											730,000	711,000	2,710,000
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.
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, or 1% of gross proceeds, as defined, for asset sales of $50,000,000 or more.
Leasing Services Fee And Commission On Asset Sales Annual Installment Maximum											4,000,000
Other Services Fee Description											Building Maintenance Services (“BMS”), our wholly-owned subsidiary, supervises the cleaning, engineering and security services at Alexander’s 731 Lexington Avenue and Kings Plaza properties for an annual fee of the costs for such services plus 6%.
Fee Income recognized by equity method investees for building maintenance services.											2,970,000	2,775,000	2,552,000
Equity Method Investment Acquisition Of Additional Common Shares														8,000,000
Acquisitions Of Equity Method Investments [Abstract]
Square Footage Of Real Estate Property																																																					1,400,000														1,200,000
Contribution Of Notes Receivable To Joint Venture																																		73,750,000																														326,250,000
Business Acquisition Cost Of Acquired Entity Cash Paid																																																															111,250,000													55,000,000	35,000,000
Business Acquisition Cost Of Acquired Entity Liabilities Incurred																																																															15,000,000
Anticipated Future Retenanting And Repositioning Costs																																																																	150,000,000
Face Value Of Mezzanine Loan Acquired																																																																												150,000,000
Face Value Of Senior Loan Acquired																																																																												35,000,000
Existing mortgage loan at time of acquisition																																																						1,215,000,000
A-Note																																																						1,100,000,000
B-Note																																																						115,000,000
Lending commitment																																																							80,000,000	110,000,000
Third party debt																																																																	710,000,000
Limited recourse and cooperation guarantee																																																					75,000,000
Non-cash purchase price accounting adjustments					0		0		13,946,000
Income Loss From Equity Method Investments	120,310,000	94,062,000	72,390,000		48,540,000		71,624,000		92,300,000		34,128,000	29,184,000	53,529,000		8,351,000		11,018,000		(25,665,000)			58,786,000		1,973,000	0			(14,881,000)	2,581,000	(1,636,000)						876,000	(10,990,000)	468,000	2,193,000	(2,419,000)	4,870,000			(1,142,000)	0	0	(16,135,000)	72,000	0	10,017,000	4,436,000	4,823,000								(18,079,000)	0	0								1,661,000	(537,000)	(19,978,000)	2,457,000	0	0			(1,461,000)	(1,155,000)	(10,395,000)	2,556,000	1,952,000	1,789,000	2,443,000	(13,677,000)	(27,715,000)
Property Management Fee Description											equal to the sum of (i) $3,000,000, (ii) 3% of the gross income from the Kings Plaza Regional Shopping Center, (iii) 2% of the gross income from the Rego Park II Shopping Center, (iv) $0.50 per square foot of the tenant-occupied office and retail space at 731 Lexington Avenue, and (v) $256,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Unpaid Balance Year End Interest Rate											1.78%
Management and leasing fees											9,115,000	9,125,000	10,765,000
Reversal of stock appreciation rights compensation expense											$ 0	$ 0	$ 24,773,000