Investments in Partially Owned Entities (Details) (USD $)	3 Months Ended								3 Months Ended			3 Months Ended				12 Months Ended			3 Months Ended					3 Months Ended					3 Months Ended				3 Months Ended					3 Months Ended			3 Months Ended			3 Months Ended		3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended					3 Months Ended		3 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Toys R Us [Member]	Jan. 28, 2012 Toys R Us [Member]	Mar. 31, 2011 Toys R Us [Member]	Jan. 29, 2011 Toys R Us [Member]	Dec. 31, 2011 Toys R Us [Member]	Oct. 29, 2011 Toys R Us [Member]	Mar. 31, 2012 Alexander's Inc. [Member]	Mar. 31, 2011 Alexander's Inc. [Member]	Dec. 31, 2011 Alexander's Inc. [Member]	Mar. 31, 2012 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Mar. 31, 2011 Lexington Realty Trust [Member]	Dec. 31, 2010 Lexington Realty Trust [Member]	Dec. 31, 2008 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Mar. 31, 2012 LNR [Member]	Dec. 31, 2011 LNR [Member]	Mar. 31, 2011 LNR [Member]	Dec. 31, 2010 LNR [Member]	Sep. 30, 2011 LNR [Member]	Mar. 31, 2012 India real estate ventures [Member]	Mar. 31, 2011 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member]	Mar. 31, 2012 India real estate ventures [Member] Range Minimum	Mar. 31, 2012 India real estate ventures [Member] Range Maximum	Mar. 31, 2012 Partially owned office buildings [Member]	Mar. 31, 2011 Partially owned office buildings [Member]	Mar. 31, 2012 Partially owned office buildings [Member] 280 Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Mar. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Minimum	Mar. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Maximum	Mar. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Mar. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Mar. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Mar. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Mar. 31, 2012 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Mar. 31, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Mar. 31, 2011 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Mar. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Warner Building [Member]	Mar. 31, 2011 Partially owned office buildings [Member] Warner Building [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Warner Building [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Mar. 31, 2011 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Mar. 31, 2012 Other equity method investments [Member]	Mar. 31, 2011 Other equity method investments [Member]	Mar. 31, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Mar. 31, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Mar. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2011 Other equity method investments [Member] Independence Plaza [Member]	Mar. 31, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Mar. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Mar. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Mar. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Mar. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Mar. 31, 2011 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2011 Other equity method investments [Member] Other equity investments [Member]
Condensed Financial Information For Equity Method Investments [Abstract]
Assets				$ 11,890,000,000				$ 13,221,000,000	$ 1,773,000,000		$ 1,771,000,000		$ 3,078,000,000				$ 3,164,000,000			$ 79,951,000,000			$ 128,536,000,000
Liabilities				9,894,000,000				11,530,000,000	1,410,000,000		1,408,000,000		1,857,000,000				1,888,000,000			79,214,000,000			127,809,000,000
Noncontrolling interests				29,000,000				0	4,000,000		4,000,000		58,000,000				59,000,000			16,000,000			55,000,000
Equity				1,967,000,000				1,691,000,000	359,000,000		359,000,000		1,163,000,000				1,217,000,000			721,000,000			672,000,000
Total revenue				5,925,000,000		5,972,000,000			64,000,000	63,000,000			83,000,000		86,000,000					49,000,000		36,000,000
Net (loss) income attributable to investee				349,000,000		339,000,000			19,000,000	18,000,000			13,000,000		12,000,000					51,000,000		58,000,000
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage			32.70%						32.40%			11.90%	12.00%	12.60%					26.20%								4.00%	36.50%			49.50%					43.70%	50.40%	50.00%			30.30%			49.50%		25.00%			55.00%			20.00%			55.00%								8.30%			51.00%		50.00%			50.00%
Carrying amount of investments in partially owned entitiies			597,860,000				506,809,000		189,142,000		189,775,000	56,106,000	57,402,000						187,251,000	174,408,000				100,571,000		80,499,000					182,998,000	184,516,000	62,562,000		53,333,000			58,841,000		58,529,000	47,899,000		47,568,000	31,769,000	23,655,000	22,238,000		20,353,000	21,056,000		20,407,000	6,199,000		6,343,000	4,746,000		2,715,000	10,991,000		11,547,000			59,478,000		59,801,000	50,194,000	48,511,000	46,821,000		46,691,000	7,805,000		7,536,000	138,437,000		140,061,000
Equity in net income, before income taxes			157,387,000		179,839,000
Income tax expense			(43,203,000)		(69,018,000)
Equity in net income			114,184,000		110,821,000				6,132,000	5,719,000		930,000		720,000					13,250,000		6,277,000			(793,000)	(207,000)																														(3,010,000)	(300,000)
Management, leasing and development fees			2,287,000		2,123,000				2,262,000	2,292,000
Gain on stock issuance												0		1,452,000
Gain Loss On Tax Settlement																			0		8,977,000
Straight Line Rent Reserves And Write Off Of Tenant Improvements																																																							0	(9,022,000)
Income (Loss) from Equity Method Investments, Total	136,504,000	129,228,000	116,471,000		112,944,000				8,394,000	8,011,000		930,000		2,172,000					13,250,000		15,254,000								(4,096,000)	(4,619,000)	(5,595,000)		158,000	2,415,000				313,000	98,000		331,000	(1,228,000)		1,715,000		794,000	619,000		683,000	723,000		(12,000)	(13,000)		(3,010,000)	(9,322,000)		527,000	2,089,000		2,348,000	(4,327,000)	(323,000)	(1,794,000)		1,682,000		(334,000)	(506,000)		362,000	131,000		961,000	(2,158,000)
Other Financial Data For Equity Method Investments [Abstract]
Number of shares owned									1,654,068			18,468,969
Closing share price									$ 393.88			$ 8.99
Equity Method Investment Market Value									651,504,000			166,036,000
Excess of investee's carrying amount over equity in net assets									58,833,000			45,082,000
Fees owed by related party under management agreements									40,685,000
Equity Method Investment Fair Value In Excess Of Carrying Amount									462,362,000			109,930,000
Non-cash impairment charges																$ 107,882,000
Equity Method Investment Acquisition Of Additional Common Shares																		8,000,000
Percentage of annual income earned in the fourth quarter	more than 80%