Investments in Partially Owned Entities (Details) (USD $)	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		6 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			3 Months Ended		6 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			3 Months Ended		6 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		3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Toys R Us [Member]	Apr. 28, 2012 Toys R Us [Member]	Jun. 30, 2011 Toys R Us [Member]	Apr. 30, 2011 Toys R Us [Member]	Jun. 30, 2012 Toys R Us [Member]	Apr. 28, 2012 Toys R Us [Member]	Jun. 30, 2011 Toys R Us [Member]	Apr. 30, 2011 Toys R Us [Member]	Dec. 31, 2011 Toys R Us [Member]	Oct. 29, 2011 Toys R Us [Member]	Jun. 30, 2012 Alexander's Inc. [Member]	Jun. 30, 2011 Alexander's Inc. [Member]	Jun. 30, 2012 Alexander's Inc. [Member]	Jun. 30, 2011 Alexander's Inc. [Member]	Dec. 31, 2011 Alexander's Inc. [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Mar. 31, 2012 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Mar. 31, 2011 Lexington Realty Trust [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Mar. 31, 2012 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Mar. 31, 2011 Lexington Realty Trust [Member]	Dec. 31, 2008 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Jun. 30, 2012 LNR [Member]	Mar. 31, 2012 LNR [Member]	Jun. 30, 2011 LNR [Member]	Mar. 31, 2011 LNR [Member]	Jun. 30, 2012 LNR [Member]	Mar. 31, 2012 LNR [Member]	Jun. 30, 2011 LNR [Member]	Mar. 31, 2011 LNR [Member]	Dec. 31, 2011 LNR [Member]	Sep. 30, 2011 LNR [Member]	Jun. 30, 2012 India real estate ventures [Member]	Jun. 30, 2011 India real estate ventures [Member]	Jun. 30, 2012 India real estate ventures [Member]	Jun. 30, 2011 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member]	Jun. 30, 2012 India real estate ventures [Member] Range Minimum	Jun. 30, 2012 India real estate ventures [Member] Range Maximum	Jun. 30, 2012 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Jun. 30, 2012 Partially owned office buildings [Member]	Jun. 30, 2011 Partially owned office buildings [Member]	Jun. 30, 2012 Partially owned office buildings [Member]	Jun. 30, 2011 Partially owned office buildings [Member]	Jun. 30, 2012 Partially owned office buildings [Member] 280 Park Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] 280 Park Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Minimum	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Maximum	Jun. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Jun. 30, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Jun. 30, 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]	Jun. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Warner Building [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Warner Building [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Warner Building [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Warner Building [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Warner Building [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Jun. 30, 2011 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Jun. 30, 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]	Jun. 30, 2012 Other equity method investments [Member]	Jun. 30, 2011 Other equity method investments [Member]	Jun. 30, 2012 Other equity method investments [Member]	Jun. 30, 2011 Other equity method investments [Member]	Jun. 30, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Jun. 30, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Jun. 30, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Jun. 30, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Jun. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2011 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2011 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 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						$ 11,889,000,000				$ 11,889,000,000				$ 13,221,000,000	$ 1,761,000,000		$ 1,761,000,000		$ 1,771,000,000		$ 3,047,000,000				$ 3,047,000,000					$ 3,164,000,000			$ 86,155,000,000				$ 86,155,000,000				$ 128,536,000,000
Liabilities						9,969,000,000				9,969,000,000				11,530,000,000	1,397,000,000		1,397,000,000		1,408,000,000		1,844,000,000				1,844,000,000					1,888,000,000			85,383,000,000				85,383,000,000				127,809,000,000
Noncontrolling interests						34,000,000				34,000,000				0	5,000,000		5,000,000		4,000,000		60,000,000				60,000,000					59,000,000			14,000,000				14,000,000				55,000,000
Equity						1,886,000,000				1,886,000,000				1,691,000,000	359,000,000		359,000,000		359,000,000		1,143,000,000				1,143,000,000					1,217,000,000			758,000,000				758,000,000				672,000,000
Total revenue						2,612,000,000		2,636,000,000		8,537,000,000		8,608,000,000			64,000,000	62,000,000	127,000,000	125,000,000			83,000,000		80,000,000		166,000,000		160,000,000						55,000,000		47,000,000		104,000,000		83,000,000
Net (loss) income attributable to investee						(66,000,000)		(77,000,000)		283,000,000		262,000,000			19,000,000	20,000,000	38,000,000	38,000,000			4,000,000		(17,000,000)		17,000,000		(5,000,000)						36,000,000		42,000,000		87,000,000		100,000,000
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage					32.50%		32.70%		32.50%		32.70%		32.70%		32.40%		32.40%			11.90%		11.70%		11.90%		11.70%			12.00%			26.20%				26.20%											4.00%	36.50%	25.00%					49.50%		49.50%								43.70%	50.40%	50.00%		50.00%			30.30%		30.30%			49.50%	49.50%		25.00%		25.00%			55.00%		55.00%			20.00%		20.00%			55.00%		55.00%												8.30%		8.30%			51.00%	51.00%		50.00%		50.00%			50.00%		50.00%
Carrying amount of investments in partially owned entitiies					573,292,000				573,292,000				506,809,000		188,709,000		188,709,000		189,775,000	53,555,000				53,555,000					57,402,000			192,788,000				192,788,000				174,408,000		96,518,000		96,518,000		80,499,000								186,102,000		186,102,000		184,516,000	62,552,000		62,552,000		53,333,000			57,754,000		57,754,000		58,529,000	48,202,000		48,202,000		47,568,000	33,107,000	33,107,000	23,655,000	23,229,000		23,229,000		20,353,000	21,688,000		21,688,000		20,407,000	6,144,000		6,144,000		6,343,000	5,009,000		5,009,000		2,715,000	10,569,000		10,569,000		11,547,000					58,595,000		58,595,000		59,801,000	51,718,000	51,718,000	48,511,000	47,365,000		47,365,000		46,691,000	7,573,000		7,573,000		7,536,000	133,970,000		133,970,000		140,061,000
Equity in net income, before income taxes					(35,664,000)		(49,017,000)		121,723,000		130,822,000
Income tax expense					14,103,000		23,969,000		(29,100,000)		(45,049,000)
Equity in net income					(21,561,000)		(25,048,000)		92,623,000		85,773,000				5,941,000	6,351,000	12,073,000	12,070,000		(236,000)		346,000		694,000		1,066,000						9,469,000		4,983,000		22,719,000		11,260,000																																																								(1,589,000)	(3,225,000)	(4,599,000)	(3,525,000)
Management, leasing and development fees					2,371,000		2,202,000		4,658,000		4,325,000				1,907,000	1,900,000	3,796,000	3,787,000
Gain on stock issuance																				0		8,308,000		0		9,760,000
Net gains from asset sales and tax settlement gains																																		6,020,000				14,997,000
Straight Line Rent Reserves And Write Off Of Tenant Improvements																																																																																														0	0	0	(9,022,000)
Income (Loss) from Equity Method Investments, Total	(6,627,000)	3,170,000	129,504,000	131,993,000	(19,190,000)		(22,846,000)		97,281,000		90,098,000				7,848,000	8,251,000	15,869,000	15,857,000		(236,000)		8,654,000		694,000		10,826,000						9,469,000		11,003,000		22,719,000		26,257,000				(3,815,000)	205,000	(4,608,000)	(2,000)					120,000	(2,364,000)	(3,976,000)	(6,983,000)	(1,955,000)	(2,184,000)	(7,550,000)	(2,184,000)		145,000	(195,000)	303,000	2,220,000				252,000	238,000	565,000	336,000		303,000	(243,000)	634,000	(1,471,000)		1,785,000	3,500,000		18,000	506,000	812,000	1,125,000		646,000	700,000	1,329,000	1,423,000		(40,000)	42,000	(52,000)	29,000		(1,589,000)	(3,225,000)	(4,599,000)	(12,547,000)		555,000	1,997,000	1,082,000	4,086,000		(823,000)	267,000	1,525,000	(4,060,000)	(289,000)	585,000	(612,000)	(1,209,000)		1,733,000	3,415,000		(500,000)	(242,000)	(834,000)	(748,000)		298,000	826,000	660,000	957,000		(2,065,000)	(902,000)	(1,104,000)	(3,060,000)
Other Financial Data For Equity Method Investments [Abstract]
Minimum Percentage of Annual Income Earned in Fourth Quarter					80.00%				80.00%
Number of shares owned															1,654,068		1,654,068			18,468,969				18,468,969
Closing share price															$ 431.11		$ 431.11			$ 8.47				$ 8.47
Equity Method Investment Market Value															713,085,000		713,085,000			156,432,000				156,432,000
Excess of investee's carrying amount over equity in net assets															58,552,000		58,552,000			45,263,000				45,263,000
Fees owed by related party under management agreements															40,480,000		40,480,000
Equity Method Investment Fair Value In Excess Of Carrying Amount															524,376,000		524,376,000			102,877,000				102,877,000
Non-cash impairment charges																												$ 107,882,000
Equity Method Investment Acquisition Of Additional Common Shares																															8,000,000