Investments in Partially Owned Entities (Details) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended				9 Months Ended						3 Months Ended				9 Months Ended				12 Months Ended			3 Months Ended			9 Months Ended				3 Months Ended				3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Alexanders Inc [Member]	Sep. 30, 2010 Alexanders Inc [Member]	Sep. 30, 2011 Alexanders Inc [Member]	Sep. 30, 2010 Alexanders Inc [Member]	Dec. 31, 2010 Alexanders Inc [Member]	Sep. 30, 2011 Toys R Us [Member]	Jul. 30, 2011 Toys R Us [Member]	Sep. 30, 2010 Toys R Us [Member]	Jul. 31, 2010 Toys R Us [Member]	Sep. 30, 2011 Toys R Us [Member]	Jul. 30, 2011 Toys R Us [Member]	Sep. 30, 2010 Toys R Us [Member]	Jul. 31, 2010 Toys R Us [Member]	Dec. 31, 2010 Toys R Us [Member]	Oct. 30, 2010 Toys R Us [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Jun. 30, 2010 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Jun. 30, 2010 Lexington Realty Trust [Member]	Dec. 31, 2008 Lexington Realty Trust [Member]	Dec. 31, 2010 Lexington Realty Trust [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Sep. 30, 2011 LNR [Member]	Jun. 30, 2011 LNR [Member]	Sep. 30, 2010 LNR [Member]	Sep. 30, 2011 LNR [Member]	Jun. 30, 2011 LNR [Member]	Sep. 30, 2010 LNR [Member]	Dec. 31, 2010 LNR [Member]	Mar. 16, 2011 Mezzanine Loans 280 Park Avenue [Member] sqft	Sep. 30, 2011 Mezzanine Loans 280 Park Avenue [Member]	Mar. 16, 2011 Mezzanine Loans 280 Park Avenue [Member] Vornado Realty Trust [Member]	Mar. 16, 2011 Mezzanine Loans 280 Park Avenue [Member] SL Green [Member]	Sep. 30, 2011 Partially owned office buildings [Member]	Sep. 30, 2010 Partially owned office buildings [Member]	Sep. 30, 2011 Partially owned office buildings [Member]	Sep. 30, 2010 Partially owned office buildings [Member]	Dec. 31, 2010 Partially owned office buildings [Member]	Sep. 30, 2011 India real estate ventures [Member]	Sep. 30, 2010 India real estate ventures [Member]	Sep. 30, 2011 India real estate ventures [Member]	Sep. 30, 2010 India real estate ventures [Member]	Dec. 31, 2010 India real estate ventures [Member]	Sep. 30, 2011 Other equity method investments [Member]	Sep. 30, 2010 Other equity method investments [Member]	Sep. 30, 2011 Other equity method investments [Member]	Sep. 30, 2010 Other equity method investments [Member]	Dec. 31, 2010 Other equity method investments [Member]	Sep. 30, 2011 Independence Plaza [Member]	Sep. 30, 2011 Vornado Realty Trust [Member]	Sep. 30, 2011 Vornado Realty Trust and SL Green [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method ownership percentage						32.40%		32.40%			32.70%				32.70%						11.70%		13.80%		11.70%		13.80%					26.20%			26.20%					49.50%																		51.00%		99.00%
Toy's fourth quarter fiscal year net income															more than 80%
Assets						$ 1,785,000,000		$ 1,785,000,000		$ 1,679,000,000		$ 11,778,000,000				$ 11,778,000,000				$ 12,810,000,000		$ 3,240,000,000	$ 3,385,000,000			$ 3,240,000,000	$ 3,385,000,000						$ 139,731,000,000	$ 143,266,000,000		$ 139,731,000,000	$ 143,266,000,000
Liabilities						1,427,000,000		1,427,000,000		1,335,000,000		9,969,000,000				9,969,000,000				11,317,000,000		1,910,000,000	2,115,000,000			1,910,000,000	2,115,000,000						139,025,000,000	142,720,000,000		139,025,000,000	142,720,000,000
Noncontrolling interests						4,000,000		4,000,000		3,000,000												60,000,000	71,000,000			60,000,000	71,000,000						43,000,000	37,000,000		43,000,000	37,000,000
Equity						354,000,000		354,000,000		341,000,000		1,809,000,000				1,809,000,000				1,493,000,000		1,270,000,000	1,199,000,000			1,270,000,000	1,199,000,000						663,000,000	509,000,000		663,000,000	509,000,000
Total revenue						65,000,000	61,000,000	190,000,000	179,000,000			2,648,000,000		2,565,000,000		11,256,000,000		11,030,000,000				83,000,000		81,000,000		252,000,000		249,000,000					73,000,000			156,000,000
Net (loss) income attributable to investee						20,000,000	18,000,000	59,000,000	49,000,000			(36,000,000)		(16,000,000)		227,000,000		292,000,000				(44,000,000)		(24,000,000)		(49,000,000)		(97,000,000)					52,000,000			152,000,000
Fees owed by Alexander's under management agreements						43,308,000		43,308,000
Closing share price						$ 361.02		$ 361.02													$ 6.54				$ 6.54
Equity Method Investment Market Value						597,152,000		597,152,000													120,787,000				120,787,000
Excess of investee's fair value over carrying amount						406,875,000		406,875,000													59,472,000				59,472,000
Excess of investee's carrying amount over equity in net assets						59,188,000		59,188,000													48,861,000				48,861,000
Non-cash impairment charges																													107,882,000
Ownership common shares, investee						1,654,068		1,654,068													18,468,969				18,468,969
Equity Method Investment Acquisition Of Additional Common Shares																															8,000,000
Carrying amount of investments in partially owned entities	1,703,584,000		1,703,584,000		1,375,006,000	190,277,000		190,277,000		186,811,000	546,258,000				546,258,000				447,334,000		61,315,000				61,315,000					57,270,000		156,090,000			156,090,000			132,973,000					452,284,000		452,284,000		181,838,000	101,155,000		101,155,000		127,193,000	196,205,000		196,205,000		241,587,000
Equity in net income, before income taxes											(26,773,000)		(32,574,000)		104,049,000		93,662,000
Income tax expense											15,135,000		27,501,000		(29,914,000)		1,914,000
Interest and other income											2,334,000		2,516,000		6,659,000		6,733,000
Equity in net income						6,437,000	5,612,000	18,507,000	14,309,000		(11,638,000)		(5,073,000)		74,135,000		95,576,000
Management and leasing fees						2,170,000	1,945,000	6,749,000	6,774,000
Income Loss From Equity Method Investments	4,248,000	(4,553,000)	137,033,000	116,109,000		8,607,000	7,557,000	25,256,000	21,083,000		(9,304,000)		(2,557,000)		80,794,000		102,309,000				(617,000)		(2,301,000)		10,209,000		3,316,000					13,656,000		0	39,913,000		0						(6,839,000)	86,000	(13,829,000)	1,864,000		(690,000)	(195,000)	(692,000)	2,062,000		(565,000)	(7,143,000)	(4,618,000)	(14,525,000)
Gain on stock issuance																					8,308,000				9,760,000		5,998,000
Gain Loss On Tax Settlement																																			8,977,000
Net Gains From Asset Sales																																6,020,000			6,020,000
Straight Line Rent Reserves And Write Off Of Tenant Improvements																																													9,022,000
Assets In Consolidated Trusts																																	138,500,000,000			138,500,000,000
Net gain on disposition of wholly owned and partially owned assets	1,298,000	5,072,000	7,975,000	12,759,000
Square Footage Of Real Estate Property																																							1,200,000
Business Acquisition Cost Of Acquired Entity Cash Paid																																							111,250,000																			55,000,000	35,000,000
Contribution Of Notes Receivable To Joint Venture																																									73,750,000	326,250,000
Business Acquisition Cost Of Acquired Entity Liabilities Incurred																																							15,000,000
AnticipatedFutureRetenantingAndRepositioningCosts																																								150,000,000
Face Value Of Mezzanine Loan Acquired																																																										150,000,000
Face Value Of Senior Loan Acquired																																																										$ 35,000,000