Investments in Partially Owned Entities (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended				6 Months Ended						3 Months Ended				6 Months Ended				12 Months Ended				3 Months Ended			6 Months Ended					3 Months Ended				3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 10, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Alexanders Inc [Member]	Jun. 30, 2010 Alexanders Inc [Member]	Jun. 30, 2011 Alexanders Inc [Member]	Jun. 30, 2010 Alexanders Inc [Member]	Dec. 31, 2010 Alexanders Inc [Member]	Jun. 30, 2011 Toys R Us [Member]	Apr. 30, 2011 Toys R Us [Member]	Jun. 30, 2010 Toys R Us [Member]	May 01, 2010 Toys R Us [Member]	Jun. 30, 2011 Toys R Us [Member]	Apr. 30, 2011 Toys R Us [Member]	Jun. 30, 2010 Toys R Us [Member]	May 01, 2010 Toys R Us [Member]	Dec. 31, 2010 Toys R Us [Member]	Oct. 30, 2010 Toys R Us [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Mar. 31, 2011 Lexington Realty Trust [Member]	Jun. 30, 2010 Lexington Realty Trust [Member]	Mar. 31, 2010 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Mar. 31, 2011 Lexington Realty Trust [Member]	Jun. 30, 2010 Lexington Realty Trust [Member]	Mar. 31, 2010 Lexington Realty Trust [Member]	Dec. 31, 2008 Lexington Realty Trust [Member]	Dec. 31, 2010 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Jun. 30, 2011 LNR [Member]	Mar. 31, 2011 LNR [Member]	Jun. 30, 2010 LNR [Member]	Jun. 30, 2011 LNR [Member]	Mar. 31, 2011 LNR [Member]	Jun. 30, 2010 LNR [Member]	Dec. 31, 2010 LNR [Member]	Sep. 30, 2010 LNR [Member]	Mar. 16, 2011 Mezzanine Loans 280 Park Avenue [Member] sqft	Jun. 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]	Jun. 30, 2011 Partially owned office buildings [Member]	Jun. 30, 2010 Partially owned office buildings [Member]	Jun. 30, 2011 Partially owned office buildings [Member]	Jun. 30, 2010 Partially owned office buildings [Member]	Dec. 31, 2010 Partially owned office buildings [Member]	Jun. 30, 2011 India real estate ventures [Member]	Jun. 30, 2010 India real estate ventures [Member]	Jun. 30, 2011 India real estate ventures [Member]	Jun. 30, 2010 India real estate ventures [Member]	Dec. 31, 2010 India real estate ventures [Member]	Jun. 30, 2011 Other equity method investments [Member]	Jun. 30, 2010 Other equity method investments [Member]	Jun. 30, 2011 Other equity method investments [Member]	Jun. 30, 2010 Other equity method investments [Member]	Dec. 31, 2010 Other equity method investments [Member]	Jun. 30, 2011 Independence Plaza [Member]	Jun. 30, 2011 Vornado Realty Trust [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method ownership percentage							3240.00%		3240.00%			3270.00%				3270.00%						1170.00%				1170.00%								2620.00%			2620.00%						4800.00%
Toy's fourth quarter fiscal year net income																more than 80%
Assets							$ 1,772,000,000		$ 1,772,000,000		$ 1,679,000,000		$ 11,951,000,000				$ 11,951,000,000				$ 12,810,000,000		$ 3,223,000,000				$ 3,223,000,000					$ 3,385,000,000			$ 141,759,000,000			$ 141,759,000,000			$ 143,266,000,000
Liabilities							1,421,000,000		1,421,000,000		1,335,000,000		10,115,000,000				10,115,000,000				11,317,000,000		1,904,000,000				1,904,000,000					2,115,000,000			141,118,000,000			141,118,000,000			142,720,000,000
Noncontrolling interests							2,000,000		2,000,000		3,000,000												76,000,000				76,000,000					71,000,000			20,000,000			20,000,000			37,000,000
Equity							349,000,000		349,000,000		341,000,000		1,836,000,000				1,836,000,000				1,493,000,000		1,243,000,000				1,243,000,000					1,199,000,000			621,000,000			621,000,000			509,000,000
Total revenue							62,000,000	59,000,000	125,000,000	118,000,000			2,636,000,000		2,608,000,000		8,608,000,000		8,465,000,000				83,000,000		82,000,000		169,000,000		168,000,000						47,000,000			83,000,000
Net (loss) income attributable to investee							20,000,000	15,000,000	38,000,000	31,000,000			(77,000,000)		(71,000,000)		262,000,000		308,000,000				(17,000,000)		(27,000,000)		(5,000,000)		(73,000,000)						42,000,000			100,000,000
Fees owed by Alexander's under management agreements							43,316,000		43,316,000
Closing share price							$ 397.00		$ 397.00													$ 9.13				$ 9.13
Equity Method Investment Market Value							656,665,000		656,665,000													168,622,000				168,622,000
Excess of investee's fair value over carrying amount							467,479,000		467,479,000													104,583,000				104,583,000
Excess of investee's carrying amount over equity in net assets							59,367,000		59,367,000													43,446,000				43,446,000
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		1,719,047,000		1,719,047,000		1,375,006,000	189,186,000		189,186,000		186,811,000	558,755,000				558,755,000				447,334,000		64,039,000				64,039,000					57,270,000			158,269,000			158,269,000			132,973,000						445,669,000		445,669,000		181,838,000	103,488,000		103,488,000		127,193,000	199,641,000		199,641,000		241,587,000
Equity in net income, before income taxes												(49,017,000)		(47,314,000)		130,822,000		126,236,000
Income tax expense												23,969,000		24,123,000		(45,049,000)		(25,587,000)
Interest and other income												2,202,000		2,187,000		4,325,000		4,217,000
Equity in net income							6,351,000	4,920,000	12,070,000	8,697,000		(25,048,000)		(23,191,000)		85,773,000		100,649,000
Management and leasing fees							2,287,000	2,146,000	4,579,000	4,829,000
Income Loss From Equity Method Investments		3,557,000	(16,552,000)	132,785,000	120,662,000		8,638,000	7,066,000	16,649,000	13,526,000		(22,846,000)		(21,004,000)		90,098,000		104,866,000				8,654,000		(428,000)		10,826,000		5,617,000						11,003,000		0	26,257,000		0							(2,366,000)	1,023,000	(6,990,000)	1,778,000		205,000	606,000	(2,000)	2,257,000		269,000	(3,815,000)	(4,053,000)	(7,382,000)
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																																			141,000,000,000			141,000,000,000
Spread Over LIBOR	170.00%
Net gain on disposition of wholly owned and partially owned assets		0	4,382,000	6,677,000	7,687,000
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																																												73,750,000,000	326,250,000,000
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