Investments in Partially Owned Entities - Debt (Details) (USD $)											0 Months Ended																											9 Months Ended						9 Months Ended														3 Months Ended					3 Months Ended											9 Months Ended												9 Months Ended			9 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Alexanders Inc [Member]	Dec. 31, 2010 Alexanders Inc [Member]	Sep. 30, 2011 Alexanders Inc [Member] Lexington Avenue Mortgage Office Space [Member]	Dec. 31, 2010 Alexanders Inc [Member] Lexington Avenue Mortgage Office Space [Member]	Sep. 30, 2011 Alexanders Inc [Member] Lexington Avenue Mortgage Retail Space [Member]	Dec. 31, 2010 Alexanders Inc [Member] Lexington Avenue Mortgage Retail Space [Member]	Sep. 30, 2011 Alexanders Inc [Member] Rego Park Construction [Member]	Dec. 31, 2010 Alexanders Inc [Member] Rego Park Construction [Member]	Jun. 10, 2011 Alexanders Inc [Member] Kings Plaza Regional Shopping Center Mortgage [Member]	Sep. 30, 2011 Alexanders Inc [Member] Kings Plaza Regional Shopping Center Mortgage [Member]	Dec. 31, 2010 Alexanders Inc [Member] Kings Plaza Regional Shopping Center Mortgage [Member]	Sep. 30, 2011 Alexanders Inc [Member] Rego Park Mortgage [Member]	Dec. 31, 2010 Alexanders Inc [Member] Rego Park Mortgage [Member]	Sep. 30, 2011 Alexanders Inc [Member] Paramus Mortgage [Member]	Dec. 31, 2010 Alexanders Inc [Member] Paramus Mortgage [Member]	Sep. 30, 2011 Toys R Us [Member]	Dec. 31, 2010 Toys R Us [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Unsecured Notes At 1075 Due 2017 [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Unsecured Notes At 1075 Due 2017 [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Unsecured Notes At 850 Due 2017 [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Unsecured Notes At 850 Due 2017 [Member]	Sep. 30, 2011 Toys R Us [Member] Secured Term Loan Facility 700 Mllion [Member]	Dec. 31, 2010 Toys R Us [Member] Secured Term Loan Facility 700 Mllion [Member]	Sep. 30, 2011 Toys R Us [Member] Credit Facility 1.85 Billion [Member]	Dec. 31, 2010 Toys R Us [Member] Credit Facility 1.85 Billion [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Uk Real Estate Facility [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Uk Real Estate Facility [Member]	Sep. 30, 2011 Toys R Us [Member] Bonds 7625 [Member]	Dec. 31, 2010 Toys R Us [Member] Bonds 7625 [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Notes 7875 [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Notes 7875 [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Secured Notes7375 [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Secured Notes7375 [Member]	Sep. 30, 2011 Toys R Us [Member] Senior Notes 7375 [Member]	Dec. 31, 2010 Toys R Us [Member] Senior Notes 7375 [Member]	Sep. 30, 2011 Toys R Us [Member] Japan Bank Loans [Member]	Dec. 31, 2010 Toys R Us [Member] Japan Bank Loans [Member]	Sep. 30, 2011 Toys R Us [Member] Spanish Real Estate Facility [Member]	Dec. 31, 2010 Toys R Us [Member] Spanish Real Estate Facility [Member]	Sep. 30, 2011 Toys R Us [Member] Japan Borrowings [Member]	Dec. 31, 2010 Toys R Us [Member] Japan Borrowings [Member]	Sep. 30, 2011 Toys R Us [Member] Junior Uk Real Estate Facility [Member]	Dec. 31, 2010 Toys R Us [Member] Junior Uk Real Estate Facility [Member]	Sep. 30, 2011 Toys R Us [Member] French Real Estate Facility [Member]	Dec. 31, 2010 Toys R Us [Member] French Real Estate Facility [Member]	Sep. 30, 2011 Toys R Us [Member] European And Australian Asset Based Revolving Credit Facility [Member]	Dec. 31, 2010 Toys R Us [Member] European And Australian Asset Based Revolving Credit Facility [Member]	Sep. 30, 2011 Toys R Us [Member] Debentures 8750 [Member]	Dec. 31, 2010 Toys R Us [Member] Debentures 8750 [Member]	Sep. 30, 2011 Toys R Us [Member] Secured Term Loan Facility 400 Million [Member]	Dec. 31, 2010 Toys R Us [Member] Secured Term Loan Facility 400 Million [Member]	Sep. 30, 2011 Toys R Us [Member] Other Investments [Member]	Dec. 31, 2010 Toys R Us [Member] Other Investments [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Jun. 30, 2011 Lexington Realty Trust [Member] Lexington Mortgage Notes Payable [Member]	Sep. 30, 2011 Lexington Realty Trust [Member] Lexington Mortgage Notes Payable [Member]	Dec. 31, 2010 Lexington Realty Trust [Member] Lexington Mortgage Notes Payable [Member]	Sep. 30, 2011 LNR [Member]	Dec. 31, 2010 LNR [Member]	Jun. 30, 2011 LNR [Member] LNR mortgage notes payable [Member]	Sep. 30, 2011 LNR [Member] LNR mortgage notes payable [Member]	Dec. 31, 2010 LNR [Member] LNR mortgage notes payable [Member]	Sep. 30, 2011 LNR [Member] Liabilities Of Consolidated Cmbs And Cdo Trusts [Member]	Dec. 31, 2010 LNR [Member] Liabilities Of Consolidated Cmbs And Cdo Trusts [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Warner Building [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Warner Building [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Madison Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Madison Avenue [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Kaempfer Properties [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Kaempfer Properties [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2011 Partially owned office buildings [Member] West 34th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] West 34th Street [Member]	Sep. 30, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2011 Partially owned office buildings [Member] Seventh Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Seventh Avenue [Member]	Sep. 30, 2011 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Dec. 31, 2010 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Sep. 30, 2011 India real estate ventures [Member] India Property Fund Lp [Member]	Sep. 30, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2010 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Sep. 30, 2011 Other equity method investments [Member] Green Courte Real Estate Partners Llc [Member]	Dec. 31, 2010 Other equity method investments [Member] Green Courte Real Estate Partners Llc [Member]	Sep. 30, 2011 Other equity method investments [Member] Waterfront Associates Llc [Member]	Dec. 31, 2010 Other equity method investments [Member] Waterfront Associates Llc [Member]	Sep. 30, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2010 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2011 Other equity method investments [Member] Wells Kinzie Garage [Member]	Dec. 31, 2010 Other equity method investments [Member] Wells Kinzie Garage [Member]	Sep. 30, 2011 Other equity method investments [Member] Orleans Hubbard Garage [Member]	Dec. 31, 2010 Other equity method investments [Member] Orleans Hubbard Garage [Member]	Sep. 30, 2011 Other equity method investments [Member] Other Equity Method Investments Other [Member]	Dec. 31, 2010 Other equity method investments [Member] Other Equity Method Investments Other [Member]	Sep. 30, 2011 Senior Uk Real Estate Facility [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage			32.40%															32.70%																																						11.70%	13.80%				26.20%								30.30%	55.00%		25.00%				20.00%		46.00%		34.80%		50.00%		50.00%		25.00%			8.30%		8.30%		2.50%		50.00%		50.00%		50.00%
Equity method ownership percentage range minimum																																																																										2.50%														4.00%
Equity method ownership percentage range maximum																																																																										5.00%														36.50%
Debt Instrument, Interest Rate, Stated Percentage																														7.625%		7.875%		7.375%		7.375%														8.75%		5.25%
Debt Instrument, Face Amount																				$ 950,000,000		$ 725,000,000		$ 700,000,000		$ 1,850,000,000				$ 500,000,000		$ 400,000,000		$ 350,000,000		$ 400,000,000														$ 21,600,000																		$ 740,000,000
Debt Instrument Maturity Date (String)					02/14		07/15		12/11			06/16		03/12		10/11				07/17		12/17		09/16								04/13		09/16		10/18				02/13		06/12-06/13		04/13		02/13		03/16		09/21		05/18																06/16	03/16	05/16		06/15				12/14		12/11		07/22		02/14		10/14													12/17		12/17				04/13
Debt Instrument, Maturity Date Range, Start																																						03/12																				2011					2011											11/11												2011			2013		2011				02/14
Debt Instrument, Maturity Date Range, End																																						02/16																				2037					2043											12/11												2022			2025		2018				09/15
Spread Over LIBOR											1.70%
Debt Instrument, Interest Rate at Period End					5.33%		4.93%		1.42%			2.04%		0.75%		5.92%		5.02%		10.75%		8.50%		6.00%		2.91%						9.50%		7.38%		9.99%				4.51%		0.98%				4.51%		2.89%		9.17%									5.80%					4.75%		5.30%		3.93%	5.00%	6.26%		1.77%		5.86%		7.00%		1.22%		5.71%		4.94%		8.07%		11.81%			6.18%		5.57%				5.35%		5.00%		5.00%		4.61%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum																																						1.85%						6.81%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum																																						2.85%						7.84%
Equity Method Investment Debt Of Partially Owned Entities			1,336,374,000	1,246,411,000	342,928,000	351,751,000	320,000,000	320,000,000	277,200,000	277,200,000		250,000,000	151,214,000	78,246,000	78,246,000	68,000,000	68,000,000	5,469,168,000	5,880,296,000	929,773,000	928,045,000	716,325,000	715,577,000	685,595,000	689,757,000	173,000,000	519,810,000	573,207,000	561,559,000	0	495,943,000	390,135,000	386,167,000	358,147,000	350,000,000	347,238,000	343,528,000	194,376,000	180,500,000	183,857,000	179,511,000	108,149,000	141,360,000	99,850,000	98,266,000	88,696,000	86,599,000	32,852,000	25,767,000	21,080,000	21,054,000	396,082,000	0	170,806,000	156,853,000				1,728,515,000	1,927,729,000	138,872,441,000	142,509,880,000		331,411,000	508,547,000	138,541,030,000	142,001,333,000	818,564,000	250,000,000	292,700,000	292,700,000	150,000,000	150,000,000	137,460,000	139,337,000	71,176,000	71,764,000	56,680,000	56,680,000	50,150,000	50,150,000	22,060,000	22,922,000	20,205,000	20,565,000	238,957,000	196,319,000		353,346,000	581,086,000	294,292,000	296,991,000	0	217,106,000	171,755,000	164,474,000	14,856,000	15,022,000	9,403,000	9,508,000	663,162,000	418,339,000
Equity Method Investment Pro Rata Share Debt Of Partially Owned Entities	39,802,179,000	40,443,346,000
Equity Method Investment Prorata Share Debt Of Partially Owned Entities Excluding Our Pro Rata Share Of Lnr Liabilities Related To Cmbs And Cdo	3,540,451,000	3,275,917,000
Notes And Loans Payable Refinanced Amount											$ 250,000
Debt Instrument, Interest Rate After Refinancing																2.90%