Investments in Partially Owned Entities (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended															3 Months Ended		12 Months Ended				12 Months Ended																								3 Months Ended			6 Months Ended	12 Months Ended						12 Months Ended													12 Months Ended												12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Toys R Us [Member]	Nov. 02, 2013 Toys R Us [Member]	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, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [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, 2013 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2012 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2011 Toys R Us [Member] Management Fees [Member]	Dec. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2012 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2012 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2011 Toys R Us [Member] Impairment Loss [Member]	Mar. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Dec. 31, 2013 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Dec. 31, 2011 Alexanders Inc [Member]	Dec. 31, 2013 Alexanders Inc [Member] Rego Park 2 Property [Member]	Dec. 31, 2013 Alexanders Inc [Member] Lexington Avenue 731 [Member]	Dec. 31, 2013 Alexanders Inc [Member] Common Area 731 Lexington Avenue [Member]	Dec. 31, 2013 Alexanders Inc [Member] Equity In Net Income After Income Taxes [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, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [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, 2013 Alexanders Inc [Member] Gain On Sale Of Real Estate [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, 2013 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Dec. 31, 2011 Lexington Realty Trust [Member]	Dec. 31, 2013 Lexington Realty Trust [Member] Equity In Net Income After Income Taxes [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, 2013 Lexington Realty Trust [Member] Net Gain From Stock Issuance And Asset Acquisition [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]	Jun. 30, 2013 LNR [Member]	Mar. 31, 2013 LNR [Member]	Apr. 19, 2013 LNR [Member]	Apr. 19, 2013 LNR [Member]	Dec. 31, 2013 LNR [Member]	Dec. 31, 2012 LNR [Member]	Sep. 30, 2012 LNR [Member]	Dec. 31, 2011 LNR [Member]	Sep. 30, 2011 LNR [Member]	Sep. 30, 2013 LNR [Member]	Dec. 31, 2013 LNR [Member] Equity In Net Income Before Income Taxes [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, 2013 LNR [Member] Net gain on sale [Member]	Dec. 31, 2012 LNR [Member] Net gain on sale [Member]	Dec. 31, 2011 LNR [Member] Net gain on sale [Member]	Dec. 31, 2013 LNR [Member] Income Tax Benefit Asset Sales And Tax Settlement Gains [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, 2013 LNR [Member] Impairment Loss [Member]	Dec. 31, 2012 LNR [Member] Impairment Loss [Member]	Dec. 31, 2011 LNR [Member] Impairment Loss [Member]	Dec. 31, 2012 LNR [Member] Consolidated CMBS and CDO trusts [Member]	Dec. 31, 2013 India real estate ventures [Member]	Dec. 31, 2012 India real estate ventures [Member]	Dec. 31, 2011 India real estate ventures [Member]	Dec. 31, 2013 India real estate ventures [Member] Equity In Net Income After Income Taxes [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, 2013 India real estate ventures [Member] Impairment Loss [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, 2013 India real estate ventures [Member] Minimum [Member]	Dec. 31, 2013 India real estate ventures [Member] Maximum [Member]	Dec. 31, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Dec. 31, 2013 Partially owned office buildings [Member]	Dec. 31, 2012 Partially owned office buildings [Member]	Dec. 31, 2011 Partially owned office buildings [Member]	Dec. 31, 2013 Other equity method investments [Member]	Dec. 31, 2012 Other equity method investments [Member]	Dec. 31, 2011 Other equity method investments [Member]	Dec. 31, 2013 All Equity Method Investees Excluding Toys [Member]	Dec. 31, 2012 All Equity Method Investees Excluding Toys [Member]	Dec. 31, 2011 All Equity Method Investees Excluding Toys [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage				32.60%																		32.40%																																																								4.10%	36.50%	25.00%
Carrying amount of investments in partially owned entities	$ 1,249,667	$ 1,704,297	$ 1,740,459	$ 83,224		$ 478,041																$ 167,785	$ 171,013														$ 0	$ 75,542								$ 0	$ 241,377			$ 0	$ 224,724																		$ 88,467	$ 95,516											$ 621,294	$ 446,933		$ 288,897	$ 212,528		$ 1,166,443	$ 1,226,256
Pre-impairment investments in joint ventures						518,041
Non-cash impairment charges				280,757		280,757											40,000
Fair value of invesements in Toys R Us				80,062																	78,542
Comprehensive Income Net of Tax	596,567	563,141	723,903	3,162
Fees owed by Alexander's under management agreements																						43,307
Excess of investee's carrying amount over equity in net assets				162,215																		42,048
Other than Temporary Impairment Losses, Investments																162,215																														0	27,231	(66,241)		27,231
Managment And Development Agreement [Abstract]
Property Management Fee Description																						equal to the sum of (i) $2,800,000, (ii) 2% of the gross revenue 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) $272,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Management and Development fee income																						2,800					272
Development Fee Percent																						6.00%
Property Management Fee Agreement Percentage Of Income																									2.00%		3.00%
Property Management Fee Agreement Price Per Square Foot																										0.50
Leasing Agreement [Abstract]
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.
Lease Fee Percentage Of Rent One To Ten Years																						3.00%
Lease Fee Percentage Of Rent Eleven To Twenty Years																						2.00%
Lease Fee Percentage Of Rent Twenty First To Thirty Years																						1.00%
Percentage Increase Lease Fee If Broker Used																						1.00%
Asset Sale Commission Threshold																						50,000
Percentage Commissions On Sale Of Assets Under Fifty Million																						3.00%
Percentage Commissions On Sale Of Assets Over Fifty Million																						1.00%
Leasing Services Fee And Commission On Asset Sales Annual Installment Maximum																						4,000
Variable Interest Rate Basis																						one-year LIBOR
Spread Over LIBOR (in percentage)																						1.00%
Unpaid Balance Year End Interest Rate																						1.84%
Other Agreements [Abstract]
Fee Income recognized by equity method investees for building maintenance services																						2,036	2,362	2,442
Other Supervisory Fees Percent																											6.00%
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	21,773,000	122,692,000			11,756,000		12,953,000															1,458,000	1,482,000																													98,530,000			0													97,000,000
Liabilities	17,982,000	117,064,000			10,437,000		11,190,000															1,124,000	1,150,000																													97,643,000			0													97,000,000
Noncontrolling interests	96,000	88,000			75,000		44,000																																													8,000			0
Equity	3,695,000	5,540,000			1,244,000		1,719,000															334,000	332,000																													879,000			0
Total revenue	14,092,000	15,119,000	15,321,000		13,046,000		13,698,000		13,956,000													196,000	191,000	185,000																									122,222			238,000		208,000
Net (loss) income attributable to investee	(368,000)	1,091,000	199,000		(396,000)		138,000		121,000													57,000	674,000	79,000																								(80,654)	94,949			266,000		224,000
Equity Method Investment Summarized Financial Information Gain Loss On Sale Of Real Estate Assets																							600,000
Equity Method Investment, Difference Excess Of Underlying Equity Over Carrying Amount				322,255
Other Financial Data For Equity Method Investments [Abstract]
Ownership common shares, investee																						1,654,068
Income Loss From Equity Method Investments	(338,785)	423,126	118,612	(362,377)		14,859		48,540		(128,919)	45,267	39,592	7,299	9,592	8,948			(240,757)	(40,000)	0		24,402	218,391	32,430				17,721	24,709	25,013	6,681	13,748	7,417	0	179,934	0	(979)	28,740	8,351	(979)	(23)	(1,409)	0	28,763	9,760	(3,776)		(21,131)		18,731	66,270		58,786			42,186	66,270	31,409	3,776	0	0	0	0	27,377	(27,231)	0	0		(3,533)	(5,008)	(14,881)	(3,533)	(5,008)	(1,087)	0	0	(13,794)				(4,212)	(3,770)	(22,270)	(10,817)	103,644	7,656	23,592	408,267	70,072
Equity Method Investment Market Value																						545,842
Excess of investee's fair value over carrying amount																						$ 378,057
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.
Other Services Fee Description																						Building Maintenance Services (“BMS”), our wholly-owned subsidiary, supervises (i) cleaning, engineering and security services at Alexander’s 731 Lexington Avenue property and (ii) security services at Alexander’s Rego Park I and Rego Park II properties, for an annual fee of the costs for such services plus 6%.