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			0 Months Ended	3 Months Ended				9 Months Ended				12 Months Ended			1 Months Ended	3 Months Ended				9 Months Ended						3 Months Ended		9 Months Ended						3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended					3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Toys R Us [Member]	Jul. 28, 2012 Toys R Us [Member]	Sep. 30, 2011 Toys R Us [Member]	Jul. 30, 2011 Toys R Us [Member]	Sep. 30, 2012 Toys R Us [Member]	Jul. 28, 2012 Toys R Us [Member]	Sep. 30, 2011 Toys R Us [Member]	Jul. 30, 2011 Toys R Us [Member]	Dec. 31, 2011 Toys R Us [Member]	Oct. 29, 2011 Toys R Us [Member]	Sep. 30, 2012 Alexander's Inc. [Member]	Sep. 30, 2011 Alexander's Inc. [Member]	Sep. 30, 2012 Alexander's Inc. [Member]	Sep. 30, 2011 Alexander's Inc. [Member]	Dec. 31, 2011 Alexander's Inc. [Member]	Oct. 21, 2012 Alexander's Inc. [Member] Subsequent Event [Member] Kings Plaza Regional Shopping Center [Member]	Sep. 30, 2012 Lexington Realty Trust [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Sep. 30, 2012 Lexington Realty Trust [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member]	Dec. 31, 2008 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Oct. 31, 2012 Lexington Realty Trust [Member] Subsequent Event [Member]	Sep. 30, 2012 LNR [Member]	Jun. 30, 2012 LNR [Member]	Sep. 30, 2011 LNR [Member]	Jun. 30, 2011 LNR [Member]	Sep. 30, 2012 LNR [Member]	Jun. 30, 2012 LNR [Member]	Sep. 30, 2011 LNR [Member]	Jun. 30, 2011 LNR [Member]	Dec. 31, 2011 LNR [Member]	Sep. 30, 2012 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Sep. 30, 2012 India real estate ventures [Member]	Sep. 30, 2011 India real estate ventures [Member]	Sep. 30, 2012 India real estate ventures [Member]	Sep. 30, 2011 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member]	Sep. 30, 2012 India real estate ventures [Member] Range Minimum	Sep. 30, 2012 India real estate ventures [Member] Range Maximum	Sep. 30, 2012 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Sep. 30, 2012 Partially owned office buildings [Member]	Sep. 30, 2011 Partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member]	Sep. 30, 2011 Partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member] 280 Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] 280 Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 280 Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Minimum	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Range Maximum	Sep. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] 330 Madison Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Washington DC 1101 17th Street [Member]	Sep. 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]	Sep. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Warner Building [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Warner Building [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Warner Building [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Warner Building [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Warner Building [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 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]	Sep. 30, 2012 Other equity method investments [Member]	Sep. 30, 2011 Other equity method investments [Member]	Sep. 30, 2012 Other equity method investments [Member]	Sep. 30, 2011 Other equity method investments [Member]	Sep. 30, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Sep. 30, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Sep. 30, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Sep. 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]	Sep. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2011 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2011 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2011 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2011 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Sep. 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						$ 11,680,000,000				$ 11,680,000,000				$ 13,221,000,000	$ 1,765,000,000		$ 1,765,000,000		$ 1,771,000,000			$ 3,017,000,000	$ 3,164,000,000			$ 3,017,000,000	$ 3,164,000,000							$ 83,899,000,000	$ 128,536,000,000			$ 83,899,000,000	$ 128,536,000,000			$ 83,000,000,000
Liabilities						9,836,000,000				9,836,000,000				11,530,000,000	1,401,000,000		1,401,000,000		1,408,000,000			1,937,000,000	1,888,000,000			1,937,000,000	1,888,000,000							83,087,000,000	127,809,000,000			83,087,000,000	127,809,000,000
Noncontrolling interests						39,000,000				39,000,000				0	5,000,000		5,000,000		4,000,000			28,000,000	59,000,000			28,000,000	59,000,000							9,000,000	55,000,000			9,000,000	55,000,000
Equity						1,805,000,000				1,805,000,000				1,691,000,000	359,000,000		359,000,000		359,000,000			1,052,000,000	1,217,000,000			1,052,000,000	1,217,000,000							803,000,000	672,000,000			803,000,000	672,000,000
Total revenue						2,552,000,000		2,648,000,000		11,089,000,000		11,256,000,000			49,000,000	47,000,000	143,000,000	139,000,000				84,000,000		78,000,000		250,000,000		238,000,000						59,000,000		73,000,000		163,000,000		156,000,000
Net (loss) income attributable to investee						(34,000,000)		(36,000,000)		249,000,000		227,000,000			19,000,000	20,000,000	57,000,000	59,000,000				5,000,000		(44,000,000)		22,000,000		(49,000,000)						63,000,000		52,000,000		150,000,000		152,000,000
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage					32.50%				32.50%				32.70%		32.40%		32.40%				11.80%				11.80%					12.00%		10.80%	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					549,421,000				549,421,000				506,809,000		186,714,000		186,714,000		189,775,000		50,271,000				50,271,000					57,402,000			197,231,000				197,231,000				174,408,000		94,241,000		94,241,000		80,499,000								190,034,000		190,034,000		184,516,000	62,272,000		62,272,000		53,333,000			57,920,000		57,920,000		58,529,000	50,275,000		50,275,000		47,568,000	34,162,000	34,162,000	23,655,000	24,900,000		24,900,000		20,353,000	22,271,000		22,271,000		20,407,000	5,870,000		5,870,000		6,343,000	11,603,000		11,603,000		2,715,000	10,042,000		10,042,000		11,547,000					52,910,000		52,910,000		59,801,000	53,545,000		53,545,000		48,511,000	47,605,000		47,605,000		46,691,000	7,373,000		7,373,000		7,536,000	160,471,000		160,471,000		140,061,000
Equity in net income, before income taxes					(22,074,000)		(26,773,000)		99,649,000		104,049,000
Income tax expense					11,118,000		15,135,000		(17,982,000)		(29,914,000)
Equity in net income					(10,956,000)		(11,638,000)		81,667,000		74,135,000				7,137,000	6,437,000	19,210,000	18,507,000			(323,000)		(617,000)		371,000		449,000						16,600,000		13,656,000		39,319,000		24,916,000																																																								(2,839,000)	(2,783,000)	(7,438,000)	(6,308,000)
Management, leasing and development fees					2,371,000		2,334,000		7,029,000		6,659,000				1,821,000	1,758,000	5,617,000	5,545,000
Gain on stock issuance																					0		0		0		9,760,000					12,983,000
Net gains from asset sales and tax settlement gains																																			0				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	12,683,000	3,836,000	142,187,000	135,829,000	(8,585,000)		(9,304,000)		88,696,000		80,794,000				8,958,000	8,195,000	24,827,000	24,052,000			(323,000)		(617,000)		371,000		10,209,000						16,600,000		13,656,000		39,319,000		39,913,000				82,000	(690,000)	(4,526,000)	(692,000)					(306,000)	(6,839,000)	(4,282,000)	(13,822,000)	(1,717,000)	(6,461,000)	(9,267,000)	(8,645,000)		(204,000)	(60,000)	99,000	2,160,000				167,000	298,000	732,000	634,000		256,000	124,000	890,000	(1,347,000)		1,744,000	5,244,000		1,224,000	315,000	2,036,000	1,440,000		591,000	671,000	1,920,000	2,094,000		(33,000)	(22,000)	(85,000)	7,000		(2,839,000)	(2,783,000)	(7,438,000)	(15,330,000)		505,000	1,079,000	1,587,000	5,165,000		(3,743,000)	(565,000)	(2,218,000)	(4,625,000)	(5,388,000)	2,413,000	(6,000,000)	1,204,000		1,828,000	1,811,000	5,243,000	1,811,000		(38,000)	(408,000)	(872,000)	(1,156,000)		347,000	631,000	1,007,000	1,588,000		(492,000)	(5,012,000)	(1,596,000)	(8,072,000)
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															$ 427.49		$ 427.49				$ 9.66				$ 9.66
Equity Method Investment Market Value															707,098,000		707,098,000				178,410,000				178,410,000
Excess of investee's carrying amount over equity in net assets															57,292,000		57,292,000				45,445,000				45,445,000
Fees owed by related party under management agreements															39,794,000		39,794,000
Equity Method Investment Fair Value In Excess Of Carrying Amount															520,384,000		520,384,000				128,139,000				128,139,000
Disposal group including discontinued operation sales price of real estate				1,500,000,000																751,000,000
Discontinued Operation Anticipated Gain Loss On Disposal Of Discontinued Operation Net Of Tax																				181,000,000
Special Dividend Income																				202,000,000
Non-cash impairment charges																													107,882,000
Equity Method Investment Acquisition Of Additional Common Shares																															8,000,000
Units in Operating Partnership converted into common shares																																																																																																													2,015,151
Common shares issued upon conversion of Operating Partnership units																																																																																																													2,015,151
Cash received per share																																																																																																													$ 13.85
Aggregate cash received from common shares																																																																																																													27,910,000
Par value of convertible debentures																																																																																																													25,000,000
Shares sold in an underwritten public offering																																15,000,000
Public offering price per share																																$ 9.45
Impairment loss																																																																																																													$ 4,936,000