Investments in Partially Owned Entities (Details) (USD $)	3 Months Ended			3 Months Ended						3 Months Ended													3 Months Ended							3 Months Ended					1 Months Ended	3 Months Ended						3 Months Ended					3 Months Ended					3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended							3 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Toys R Us [Member]	Feb. 02, 2013 Toys R Us [Member]	Dec. 31, 2012 Toys R Us [Member]	Mar. 31, 2012 Toys R Us [Member]	Jan. 28, 2012 Toys R Us [Member]	Oct. 27, 2012 Toys R Us [Member]	Mar. 31, 2013 Toys R Us [Member] Equity In Net Income Before Income Taxes [Member]	Mar. 31, 2012 Toys R Us [Member] Equity In Net Income Before Income Taxes [Member]	Mar. 31, 2013 Toys R Us [Member] Income Tax Benefit [Member]	Mar. 31, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Mar. 31, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Mar. 31, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Mar. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Mar. 31, 2012 Toys R Us [Member] Impairment Loss [Member]	Mar. 31, 2013 Toys R Us [Member] Management Fees [Member]	Mar. 31, 2012 Toys R Us [Member] Management Fees [Member]	Mar. 31, 2013 Alexanders Inc [Member]	Mar. 31, 2012 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Mar. 31, 2013 Alexanders Inc [Member] Equity In Net Income Before Income Taxes [Member]	Mar. 31, 2012 Alexanders Inc [Member] Equity In Net Income Before Income Taxes [Member]	Mar. 31, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Mar. 31, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Mar. 31, 2013 Lexington Realty Trust [Member]	Mar. 31, 2012 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Mar. 31, 2013 LNR [Member]	Dec. 31, 2012 LNR [Member]	Mar. 31, 2012 LNR [Member]	Dec. 31, 2011 LNR [Member]	Sep. 30, 2012 LNR [Member]	Apr. 22, 2013 LNR [Member] Subsequent Event [Member]	Mar. 31, 2013 India real estate ventures [Member]	Mar. 31, 2012 India real estate ventures [Member]	Dec. 31, 2012 India real estate ventures [Member]	Mar. 31, 2013 India real estate ventures [Member] Minimum [Member]	Mar. 31, 2013 India real estate ventures [Member] Maximum [Member]	Mar. 31, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Mar. 31, 2013 Partially owned office buildings [Member]	Mar. 31, 2012 Partially owned office buildings [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Minimum [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Maximum [Member]	Mar. 31, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Mar. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Mar. 31, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Mar. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Mar. 31, 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]	Mar. 31, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Mar. 31, 2013 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Mar. 31, 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]	Mar. 31, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Mar. 31, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Mar. 31, 2013 Other equity method investments [Member]	Mar. 31, 2012 Other equity method investments [Member]	Mar. 31, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Mar. 31, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Dec. 31, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Mar. 31, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Mar. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Mar. 31, 2013 Other equity method investments [Member] Other equity investments [Member]	Mar. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Mar. 31, 2013 Other equity method investments [Member] Independence Plaza [Member]	Mar. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage				32.50%																32.40%										26.20%									4.00%	36.50%	25.00%			49.50%						43.70%	50.40%	50.00%			30.30%			49.50%			25.00%			55.00%			20.00%						55.00%				50.00%			50.00%
Fair value of invesements in Toys R Us																$ 78,542,000
Non-cash impairment charges					118,542,000	40,000,000
Carrying amount of investments in partially owned entities	1,672,482,000		1,704,297,000	474,466,000		478,041,000														171,820,000		171,013,000					0		75,542,000	241,377,000	224,724,000					94,691,000		95,516,000						199,466,000		197,516,000	61,827,000		62,627,000			56,500,000		57,033,000	52,238,000		50,509,000	37,212,000		35,527,000	31,581,000		30,277,000	10,118,000		8,775,000	5,299,000		5,368,000	8,942,000		9,315,000					46,309,000		48,122,000	7,380,000		7,205,000	173,256,000		147,187,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets					11,920,000,000				12,953,000,000											1,485,000,000		1,482,000,000									92,267,000,000			98,530,000,000
Liabilities					9,921,000,000				11,190,000,000											1,150,000,000		1,150,000,000									91,204,000,000			97,643,000,000
Noncontrolling interests					49,000,000				44,000,000																						8,000,000			8,000,000
Equity					1,950,000,000				1,719,000,000											335,000,000		332,000,000									1,055,000,000			879,000,000
Total revenue					5,770,000,000			5,925,000,000												49,000,000	47,000,000										48,000,000		49,000,000
Net (loss) income attributable to investee					241,000,000			349,000,000												14,000,000	19,000,000										176,000,000		51,000,000
Other Financial Data For Equity Method Investments [Abstract]
Ownership common shares, investee (in shares)																				1,654,068
Fees owed by Alexander's under management agreements																				45,623,000
Closing share price (in dollars per share)																				$ 329.69
Equity Method Investment Market Value																				545,330,000
Excess of investee's fair value over carrying amount																				373,510,000
Excess of investee's carrying amount over equity in net assets				(141,270,000)																43,595,000
Proceeds From Sale Of Equity Method Investments																																			241,000,000
Sales Of Real Estate																																			1,053,000,000
Income Loss From Equity Method Investments	$ 22,525,000	$ 136,131,000		$ 1,759,000			$ 116,471,000			$ 137,888,000	$ 157,387,000	$ 59,346,000	$ 43,203,000	$ 78,542,000	$ 114,184,000	$ (78,542,000)	$ 0	$ 1,759,000	$ 2,287,000	$ 6,076,000	$ 8,021,000		$ 4,589,000	$ 6,132,000	$ 1,487,000	$ 1,889,000	$ (979,000)	$ 930,000		$ 18,731,000		$ 13,250,000				$ (767,000)	$ (793,000)					$ (582,000)	$ (4,096,000)	$ (2,569,000)	$ (5,595,000)		$ (446,000)	$ 158,000				$ 172,000	$ 313,000		$ 457,000	$ 331,000		$ 2,019,000	$ 1,715,000		$ 1,304,000	$ 794,000		$ (2,346,000)	$ (3,010,000)		$ (45,000)	$ (12,000)		$ 488,000	$ 527,000		$ 384,000	$ 683,000	$ (1,713,000)	$ 2,348,000	$ (2,374,000)	$ (334,000)		$ 859,000	$ 362,000		$ (198,000)	$ 638,000		$ 0	$ 1,682,000