Investments in Partially Owned Entities (Details) (USD $)	12 Months Ended																																		0 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended					12 Months Ended						0 Months Ended	2 Months Ended	12 Months Ended				12 Months Ended										12 Months Ended									12 Months Ended																				12 Months Ended																						12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Toys R Us [Member]	Oct. 27, 2012 Toys R Us [Member]	Dec. 31, 2011 Toys R Us [Member]	Oct. 29, 2011 Toys R Us [Member]	Dec. 31, 2010 Toys R Us [Member]	Oct. 30, 2010 Toys R Us [Member]	Dec. 31, 2012 Toys R Us [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2011 Toys R Us [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2010 Toys R Us [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Dec. 31, 2011 Toys R Us [Member] Income Tax Benefit [Member]	Dec. 31, 2010 Toys R Us [Member] Income Tax Benefit [Member]	Dec. 31, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2011 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2010 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2012 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2011 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2010 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2012 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Dec. 31, 2011 Alexanders Inc [Member]	Dec. 31, 2010 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2011 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2010 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Dec. 31, 2011 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Dec. 31, 2010 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Dec. 31, 2012 Alexanders Inc [Member] Gain On Sale Of Real Estate [Member]	Dec. 31, 2011 Alexanders Inc [Member] Gain On Sale Of Real Estate [Member]	Dec. 31, 2010 Alexanders Inc [Member] Gain On Sale Of Real Estate [Member]	Nov. 16, 2012 Alexanders Inc [Member] Kings Plaza Regional Shopping Center [Member]	Dec. 31, 2012 Alexanders Inc [Member] Kings Plaza Regional Shopping Center [Member]	Dec. 31, 2012 Alexanders Inc [Member] Kings Plaza Regional Shopping Center [Member] Long Term Capital Gain [Member]	Oct. 31, 2008 Lexington Realty Trust [Member]	Sep. 30, 2012 Lexington Realty Trust [Member]	Sep. 30, 2011 Lexington Realty Trust [Member]	Sep. 30, 2010 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2011 Lexington Realty Trust [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2010 Lexington Realty Trust [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2012 Lexington Realty Trust [Member] Net Gain From Stock Issuance And Asset Acquisition [Member]	Dec. 31, 2011 Lexington Realty Trust [Member] Net Gain From Stock Issuance And Asset Acquisition [Member]	Dec. 31, 2010 Lexington Realty Trust [Member] Net Gain From Stock Issuance And Asset Acquisition [Member]	Jan. 24, 2013 LNR [Member]	Sep. 30, 2010 LNR [Member]	Sep. 30, 2012 LNR [Member]	Sep. 30, 2011 LNR [Member]	Dec. 31, 2012 LNR [Member]	Dec. 31, 2011 LNR [Member]	Dec. 31, 2012 LNR [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2011 LNR [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2010 LNR [Member] Equity In Net Income Before Income Taxes [Member]	Dec. 31, 2012 LNR [Member] Income Tax Benefit Asset Sales ANd Tax Settlement Gains [Member]	Dec. 31, 2011 LNR [Member] Income Tax Benefit Asset Sales ANd Tax Settlement Gains [Member]	Dec. 31, 2010 LNR [Member] Income Tax Benefit Asset Sales ANd Tax Settlement Gains [Member]	Dec. 31, 2012 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Dec. 31, 2011 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Dec. 31, 2012 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member]	Dec. 31, 2012 India real estate ventures [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2011 India real estate ventures [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2010 India real estate ventures [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2012 India real estate ventures [Member] Impairment Loss [Member]	Dec. 31, 2011 India real estate ventures [Member] Impairment Loss [Member]	Dec. 31, 2010 India real estate ventures [Member] Impairment Loss [Member]	Dec. 31, 2012 India real estate ventures [Member] Minimum [Member]	Dec. 31, 2012 India real estate ventures [Member] Maximum [Member]	Dec. 31, 2012 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Dec. 31, 2012 Partially owned office buildings [Member]	Dec. 31, 2011 Partially owned office buildings [Member]	Dec. 31, 2010 Partially owned office buildings [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Equity In Net Income After Income Taxes [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Straight Line Reserves And Write Off Of Tenant Improvements [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Straight Line Reserves And Write Off Of Tenant Improvements [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Straight Line Reserves And Write Off Of Tenant Improvements [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Park Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Minimum [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member] Maximum [Member]	Dec. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building And 1101 17th Street [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Washington DC Office Warner Building And 1101 17th Street [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Washington DC Office Warner Building And 1101 17th Street [Member]	Dec. 31, 2012 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]	Dec. 31, 2010 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Madison Avenue [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Madison Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2010 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member]	Dec. 31, 2011 Partially owned office buildings [Member] Mezzanine Loans 280 Park Avenue [Member]	Dec. 31, 2012 Other equity method investments [Member]	Dec. 31, 2011 Other equity method investments [Member]	Dec. 31, 2010 Other equity method investments [Member]	Sep. 30, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2012 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2011 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2010 Other equity method investments [Member] Verde Realty Operating Partnership [Member]	Dec. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2011 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2010 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2011 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2010 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2011 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2010 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2011 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2010 Other equity method investments [Member] Other equity investments [Member]
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	$ 122,692,000,000	$ 153,861,000,000			$ 12,953,000,000		$ 13,221,000,000																$ 1,482,000,000	$ 1,771,000,000															$ 3,386,000,000	$ 3,164,000,000												$ 98,530,000,000	$ 128,536,000,000									$ 97,000,000,000	$ 127,000,000,000
Liabilities	117,064,000,000	147,854,000,000			11,190,000,000		11,530,000,000																1,150,000,000	1,408,000,000															2,211,000,000	1,888,000,000												97,643,000,000	127,809,000,000									97,000,000,000	127,000,000,000
Noncontrolling interests	88,000,000	132,000,000			44,000,000		0																0	4,000,000															27,000,000	59,000,000												8,000,000	55,000,000
Equity	5,540,000,000	5,875,000,000			1,719,000,000		1,691,000,000																332,000,000	359,000,000															1,148,000,000	1,217,000,000												879,000,000	672,000,000
Total revenue	15,119,000,000	15,321,000,000	14,962,000,000		13,698,000,000		13,956,000,000		13,749,000,000														191,000,000	185,000,000	174,000,000														333,000,000	315,000,000	330,000,000										23,000,000	238,000,000	208,000,000
Net (loss) income attributable to investee	1,091,000,000	199,000,000	63,000,000		138,000,000		121,000,000		189,000,000														674,000,000	79,000,000	66,000,000														196,000,000	(81,000,000)	(90,000,000)										8,000,000	266,000,000	224,000,000
Equity Method Investment Summarized Financial Information Gain Loss On Sale Of Real Estate Assets																							600,000
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage				32.60%		32.70%																	32.40%																			10.50%	12.00%											26.20%																		4.00%	36.50%	25.00%										49.50%			50.00%						43.70%	50.40%	30.30%			55.00%						49.50%			25.00%			20.00%			55.00%		49.50%												50.00%			50.00%
Carrying amount of investments in partially owned entities	1,704,297,000	1,740,459,000	1,375,006,000	478,041,000		506,809,000																	171,013,000	189,775,000																		75,542,000	57,402,000											224,724,000	174,408,000									95,516,000	80,499,000																						57,033,000	58,529,000		62,627,000	53,333,000				50,509,000	47,568,000		0	20,407,000		9,315,000	11,547,000		35,527,000	23,655,000		30,277,000	20,353,000		5,368,000	6,343,000		8,775,000	2,715,000	197,516,000	184,516,000					0	59,801,000		0	48,511,000		48,122,000	46,691,000		7,205,000	7,536,000		147,187,000	140,061,000
Non-cash impairment charges																																						107,882,000
Impairment loss																																																																																																																								4,936,000
Sales Commission																																				6,423,000
Gain on sale of partial interest																																																																																																																																						12,525,000
Other Financial Data For Equity Method Investments [Abstract]
Subsidiary or Equity Method Investee, Price-Per-Share																							$ 330.80																			$ 10.45
Ownership common shares, investee																							1,654,068																			18,468,969
Equity Method Investment Market Value																							547,166,000																			193,001,000
Excess of investee's fair value over carrying amount																							376,153,000																			117,459,000
Excess of investee's carrying amount over equity in net assets																							43,383,000																			(31,427,000)
Development Fee Percent																							6.00%
Leasing Services Fee Description																							leasing services for a fee of 3% of rent for the first ten years of a lease term, 2% of rent for the eleventh through twentieth year of a lease term and 1% of rent for the twenty-first through thirtieth year of a lease term, subject to the payment of rents by Alexander’s tenants. In the event third-party real estate brokers are used, our fee increases by 1% and we are responsible for the fees to the third-parties.
Commission On Asset Sales Description																							commission upon the sale of any of Alexander’s assets equal to 3% of gross proceeds, as defined, for asset sales less than $50,000,000, or 1% of gross proceeds, as defined, for asset sales of $50,000,000 or more.
Leasing Services Fee And Commission On Asset Sales Annual Installment Maximum																							4,000,000
Spread Over LIBOR																							1.00%
Variable Interest Rate Basis																							one-year LIBOR
Other Services Fee Description																							Building Maintenance Services (“BMS”), our wholly-owned subsidiary, supervises the cleaning, engineering and security services at Alexander’s 731 Lexington Avenue property for an annual fee of the costs for such services plus 6%.
Fee Income recognized by equity method investees for building maintenance services.																							2,934,000	2,970,000	2,775,000
Equity Method Investment Acquisition Of Additional Common Shares																																						8,000,000
Income Loss From Equity Method Investments	423,126,000	118,612,000	92,493,000	14,859,000		48,540,000		71,624,000		28,638,000	38,460,000	16,401,000	16,629,000	1,132,000	45,418,000	45,267,000	39,592,000	61,819,000	9,592,000	8,948,000	9,805,000	(40,000,000)				24,709,000	25,013,000	20,059,000	13,748,000	7,417,000	7,556,000	179,934,000	0	0			201,796,000							(23,000)	(1,409,000)	(2,692,000)	28,763,000	9,760,000	13,710,000							66,270,000	31,409,000	1,973,000	0	27,377,000	0					(5,008,000)	(1,087,000)	2,581,000	0	(13,794,000)	0				(3,770,000)	(22,270,000)	(8,901,000)	(10,186,000)	(9,853,000)	(344,000)	0	(9,022,000)	0	(11,510,000)	(18,079,000)	0	1,014,000	876,000	(10,990,000)	822,000	2,193,000	(2,419,000)			1,123,000	(1,142,000)	0	2,576,000	2,740,000	416,000	1,905,000	7,735,000	2,405,000	7,009,000	198,000	0	3,609,000	2,126,000	2,059,000	(132,000)	(42,000)	(28,000)					103,644,000	7,656,000	(13,417,000)		(5,703,000)	1,661,000	(537,000)	111,865,000	2,457,000	0	(1,309,000)	(1,461,000)	(1,155,000)	1,429,000	2,556,000	1,952,000	(2,638,000)	2,443,000	(13,677,000)
Property Management Fee Description																							equal to the sum of (i) $2,800,000, (ii) 2% of the gross income from the Rego Park II Shopping Center, (iii) $0.50 per square foot of the tenant-occupied office and retail space at 731 Lexington Avenue, and (iv) $264,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Unpaid Balance Year End Interest Rate																							2.13%
Sales Of Real Estate																																			751,000,000
Net gains on sale of real estate	245,799,000	51,623,000	2,506,000																																179,934,000
Anticipated Net Proceeds From Sale Of Equity Method Investment																																																		$ 241,000,000
Units In Operating Partnership Converted Into Common Shares																																																																																																																									2,015,151
Common Shares Issued Upon Conversion Of Operating Partnership Units																																																																																																																									2,015,151