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		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			1 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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Toys R Us [Member]	May 04, 2013 Toys R Us [Member]	Feb. 02, 2013 Toys R Us [Member]	Dec. 31, 2012 Toys R Us [Member]	Jun. 30, 2012 Toys R Us [Member]	Apr. 28, 2012 Toys R Us [Member]	Jun. 30, 2013 Toys R Us [Member]	May 04, 2013 Toys R Us [Member]	Jun. 30, 2012 Toys R Us [Member]	Apr. 28, 2012 Toys R Us [Member]	Oct. 27, 2012 Toys R Us [Member]	Jun. 30, 2013 Toys R Us [Member] Equity in net income [Member]	Jun. 30, 2012 Toys R Us [Member] Equity in net income [Member]	Jun. 30, 2013 Toys R Us [Member] Equity in net income [Member]	Jun. 30, 2012 Toys R Us [Member] Equity in net income [Member]	Jun. 30, 2013 Toys R Us [Member] Income Tax Benefit [Member]	Jun. 30, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Jun. 30, 2013 Toys R Us [Member] Income Tax Benefit [Member]	Jun. 30, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Jun. 30, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Jun. 30, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Jun. 30, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Jun. 30, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Jun. 30, 2013 Toys R Us [Member] Impairment Loss [Member]	Jun. 30, 2012 Toys R Us [Member] Impairment Loss [Member]	Jun. 30, 2013 Toys R Us [Member] Impairment Loss [Member]	Jun. 30, 2012 Toys R Us [Member] Impairment Loss [Member]	Mar. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Jun. 30, 2013 Toys R Us [Member] Management Fees [Member]	Jun. 30, 2012 Toys R Us [Member] Management Fees [Member]	Jun. 30, 2013 Toys R Us [Member] Management Fees [Member]	Jun. 30, 2012 Toys R Us [Member] Management Fees [Member]	Jun. 30, 2013 Alexanders Inc [Member]	Jun. 30, 2012 Alexanders Inc [Member]	Jun. 30, 2013 Alexanders Inc [Member]	Jun. 30, 2012 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Jun. 30, 2013 Alexanders Inc [Member] Equity in net income [Member]	Jun. 30, 2012 Alexanders Inc [Member] Equity in net income [Member]	Jun. 30, 2013 Alexanders Inc [Member] Equity in net income [Member]	Jun. 30, 2012 Alexanders Inc [Member] Equity in net income [Member]	Jun. 30, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Jun. 30, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Jun. 30, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Jun. 30, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Jun. 30, 2013 Lexington Realty Trust [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Jun. 30, 2013 Lexington Realty Trust [Member]	Jun. 30, 2012 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Apr. 22, 2013 LNR [Member]	Jun. 30, 2013 LNR [Member]	Mar. 31, 2013 LNR [Member]	Jun. 30, 2012 LNR [Member]	Jun. 30, 2013 LNR [Member]	Jun. 30, 2012 LNR [Member]	Dec. 31, 2012 LNR [Member]	Jun. 30, 2013 India real estate ventures [Member]	Jun. 30, 2012 India real estate ventures [Member]	Jun. 30, 2013 India real estate ventures [Member]	Jun. 30, 2012 India real estate ventures [Member]	Dec. 31, 2012 India real estate ventures [Member]	Jun. 30, 2013 India real estate ventures [Member] Minimum [Member]	Jun. 30, 2013 India real estate ventures [Member] Maximum [Member]	Jun. 30, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Jun. 30, 2013 Partially owned office buildings [Member]	Jun. 30, 2012 Partially owned office buildings [Member]	Jun. 30, 2013 Partially owned office buildings [Member]	Jun. 30, 2012 Partially owned office buildings [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Minimum [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Maximum [Member]	Jun. 30, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2013 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, 2013 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]	Dec. 31, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Jun. 30, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Jun. 30, 2013 Other equity method investments [Member]	Jun. 30, 2012 Other equity method investments [Member]	Jun. 30, 2013 Other equity method investments [Member]	Jun. 30, 2012 Other equity method investments [Member]	Jun. 30, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Apr. 24, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Jun. 30, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2013 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2013 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Jun. 30, 2013 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2013 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]	Jun. 30, 2013 Other equity method investments [Member] Independence Plaza [Member] Non-controlling Interests
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage						32.60%						32.60%																										32.40%		32.40%																		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%										50.00%		50.00%		50.00%								50.10%		50.10%
Fair value of invesements in Toys R Us																																	$ 78,542,000
Non-cash impairment charges								118,542,000	40,000,000			118,542,000
Carrying amount of investments in partially owned entities	1,449,408,000		1,449,408,000		1,704,297,000	417,764,000			478,041,000			417,764,000																										170,181,000		170,181,000		171,013,000									0		0		75,542,000		0	241,000,000		0		224,724,000	90,717,000		90,717,000		95,516,000								207,956,000		207,956,000		197,516,000	60,345,000		60,345,000		62,627,000			56,696,000		56,696,000		57,033,000	54,367,000		54,367,000		50,509,000	38,664,000		38,664,000		35,527,000	32,766,000		32,766,000		30,277,000	11,754,000		11,754,000		8,775,000	5,242,000		5,242,000		5,368,000	9,508,000		9,508,000		9,315,000									0		0			48,122,000	7,248,000		7,248,000		7,205,000	119,631,000		119,631,000		147,187,000	166,569,000		166,569,000		0	26,679,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets							11,303,000,000						11,303,000,000			12,953,000,000																						1,469,000,000		1,469,000,000		1,482,000,000
Liabilities							9,475,000,000						9,475,000,000			11,190,000,000																						1,136,000,000		1,136,000,000		1,150,000,000
Noncontrolling interests							67,000,000						67,000,000			44,000,000
Equity							1,761,000,000						1,761,000,000			1,719,000,000																						333,000,000		333,000,000		332,000,000
Total revenue							2,408,000,000				2,612,000,000		8,178,000,000		8,537,000,000																							47,000,000	47,000,000	96,000,000	94,000,000
Net (loss) income attributable to investee							(119,000,000)				(66,000,000)		122,000,000		283,000,000																							13,000,000	19,000,000	27,000,000	38,000,000
Other Financial Data For Equity Method Investments [Abstract]
Ownership common shares, investee (in shares)																																						1,654,068		1,654,068
Fees owed by Alexander's under management agreements																																						44,883,000		44,883,000
Closing share price (in dollars per share)																																						$ 293.71		$ 293.71
Equity Method Investment Market Value																																						485,816,000		485,816,000
Excess of investee's fair value over carrying amount																																						315,635,000		315,635,000
Excess of investee's carrying amount over equity in net assets						146,215,000						146,215,000																										43,292,000		43,292,000
Proceeds From Sale Of Equity Method Investments																																																								241,000,000
Sales Of Real Estate																																																								1,053,000,000
Income Loss From Equity Method Investments	$ (35,389,000)	$ (6,627,000)	$ (12,864,000)	$ 129,504,000		$ (36,861,000)				$ (19,190,000)		$ (35,102,000)		$ 97,281,000			$ (64,372,000)	$ (35,664,000)	$ 73,516,000	$ 121,723,000	$ 25,664,000	$ 14,103,000	$ (33,682,000)	$ (29,100,000)	$ (38,708,000)	$ (21,561,000)	$ 39,834,000	$ 92,623,000	$ 0	$ 0	$ (78,542,000)	$ 0		$ 1,847,000	$ 2,371,000	$ 3,606,000	$ 4,658,000	$ 5,751,000	$ 7,848,000	$ 11,827,000	$ 15,869,000		$ 4,077,000	$ 5,941,000	$ 8,486,000	$ 12,073,000	$ 1,674,000	$ 1,907,000	$ 3,341,000	$ 3,796,000	$ 0	$ (236,000)	$ (979,000)	$ 694,000			$ 0	$ 18,731,000	$ 9,469,000	$ 18,731,000	$ 22,719,000		$ (414,000)	$ (3,815,000)	$ (1,181,000)	$ (4,608,000)					$ (1,042,000)	$ 120,000	$ (1,624,000)	$ (3,976,000)	$ (2,021,000)	$ (1,955,000)	$ (4,590,000)	$ (7,550,000)		$ (1,005,000)	$ 145,000	$ (1,451,000)	$ 303,000				$ 196,000	$ 252,000	$ 368,000	$ 565,000		$ (83,000)	$ 303,000	$ 374,000	$ 634,000		$ 1,899,000	$ 1,785,000	$ 3,918,000	$ 3,500,000		$ 1,185,000	$ 18,000	$ 2,489,000	$ 812,000		$ (1,996,000)	$ (1,589,000)	$ (4,342,000)	$ (4,599,000)		$ (18,000)	$ (40,000)	$ (63,000)	$ (52,000)		$ 565,000	$ 555,000	$ 1,053,000	$ 1,082,000		$ 236,000	$ 646,000	$ 620,000	$ 1,329,000	$ (2,823,000)	$ (823,000)	$ (4,536,000)	$ 1,525,000	$ 16,000	$ (500,000)	$ (2,358,000)	$ (834,000)			$ 426,000	$ 298,000	$ 1,285,000	$ 660,000		$ (2,147,000)	$ (2,354,000)	$ (2,345,000)	$ (1,716,000)		$ (1,118,000)	$ 1,733,000	$ (1,118,000)	$ 3,415,000