Investments in Partially Owned Entities (Details) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended					9 Months Ended					3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			1 Months Ended	3 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended						3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended					3 Months Ended		9 Months Ended					3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Non-controlling Interests	Sep. 30, 2013 Toys R Us [Member]	Aug. 03, 2013 Toys R Us [Member]	Dec. 31, 2012 Toys R Us [Member]	Sep. 30, 2012 Toys R Us [Member]	Jul. 28, 2012 Toys R Us [Member]	Sep. 30, 2013 Toys R Us [Member]	Aug. 03, 2013 Toys R Us [Member]	Sep. 30, 2012 Toys R Us [Member]	Jul. 28, 2012 Toys R Us [Member]	Oct. 27, 2012 Toys R Us [Member]	Sep. 30, 2013 Toys R Us [Member] Equity in net income [Member]	Sep. 30, 2012 Toys R Us [Member] Equity in net income [Member]	Sep. 30, 2013 Toys R Us [Member] Equity in net income [Member]	Sep. 30, 2012 Toys R Us [Member] Equity in net income [Member]	Sep. 30, 2013 Toys R Us [Member] Income Tax Benefit [Member]	Sep. 30, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Sep. 30, 2013 Toys R Us [Member] Income Tax Benefit [Member]	Sep. 30, 2012 Toys R Us [Member] Income Tax Benefit [Member]	Sep. 30, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2013 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2012 Toys R Us [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2013 Toys R Us [Member] Impairment Loss [Member]	Sep. 30, 2012 Toys R Us [Member] Impairment Loss [Member]	Sep. 30, 2013 Toys R Us [Member] Impairment Loss [Member]	Sep. 30, 2012 Toys R Us [Member] Impairment Loss [Member]	Mar. 31, 2013 Toys R Us [Member] Impairment Loss [Member]	Sep. 30, 2013 Toys R Us [Member] Management Fees [Member]	Sep. 30, 2012 Toys R Us [Member] Management Fees [Member]	Sep. 30, 2013 Toys R Us [Member] Management Fees [Member]	Sep. 30, 2012 Toys R Us [Member] Management Fees [Member]	Sep. 30, 2013 Alexanders Inc [Member]	Sep. 30, 2012 Alexanders Inc [Member]	Sep. 30, 2013 Alexanders Inc [Member]	Sep. 30, 2012 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Sep. 30, 2013 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2012 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2013 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2012 Alexanders Inc [Member] Equity In Net Income After Income Taxes [Member]	Sep. 30, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Sep. 30, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Sep. 30, 2013 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Sep. 30, 2012 Alexanders Inc [Member] Management Leasing And Development Fees [Member]	Sep. 30, 2013 Lexington Realty Trust [Member]	Sep. 30, 2012 Lexington Realty Trust [Member]	Sep. 30, 2013 Lexington Realty Trust [Member]	Sep. 30, 2012 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Apr. 22, 2013 LNR [Member]	Sep. 30, 2013 LNR [Member]	Mar. 31, 2013 LNR [Member]	Sep. 30, 2012 LNR [Member]	Sep. 30, 2013 LNR [Member]	Sep. 30, 2012 LNR [Member]	Dec. 31, 2012 LNR [Member]	Sep. 30, 2013 India real estate ventures [Member]	Sep. 30, 2012 India real estate ventures [Member]	Sep. 30, 2013 India real estate ventures [Member]	Sep. 30, 2012 India real estate ventures [Member]	Dec. 31, 2012 India real estate ventures [Member]	Sep. 30, 2013 India real estate ventures [Member] Minimum [Member]	Sep. 30, 2013 India real estate ventures [Member] Maximum [Member]	Sep. 30, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Sep. 30, 2013 Partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member]	Sep. 30, 2013 Partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Madison Avenue 650 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue 650 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Minimum [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member] Maximum [Member]	Sep. 30, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Sep. 30, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Sep. 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]	Sep. 30, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Other partially owned office buildings [Member]	Sep. 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]	Sep. 30, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Sep. 30, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Sep. 30, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Sep. 30, 2013 Other equity method investments [Member]	Sep. 30, 2012 Other equity method investments [Member]	Sep. 30, 2013 Other equity method investments [Member]	Sep. 30, 2012 Other equity method investments [Member]	Sep. 30, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2012 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 30, 2013 Other equity method investments [Member] Downtown Crossing Boston [Member]	Sep. 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]	Sep. 30, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Sep. 30, 2013 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2013 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2012 Other equity method investments [Member] Other equity investments [Member]	Dec. 31, 2012 Other equity method investments [Member] Other equity investments [Member]	Sep. 30, 2013 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2013 Other equity method investments [Member] Independence Plaza [Member]	Sep. 30, 2012 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]	Sep. 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%			20.10%							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%
Toys fourth quarter net income as a percentage of its fiscal year net income											80.00%
Fair value of invesements in Toys R Us																																$ 78,542,000
Non-cash impairment charges								40,000,000			118,542,000
Carrying amount of investments in partially owned entities						378,615,000		478,041,000			378,615,000																										167,618,000		167,618,000		171,013,000									0		0		75,542,000		0	241,000,000		0		224,724,000	90,200,000		90,200,000		95,516,000								229,152,000		229,152,000		197,516,000	121,775,000	0	59,416,000		59,416,000		62,627,000			56,743,000		56,743,000		57,033,000	56,064,000		56,064,000		50,509,000	40,047,000		40,047,000		35,527,000	33,991,000		33,991,000		30,277,000	13,387,000		13,387,000		8,775,000	5,191,000		5,191,000		5,368,000	9,338,000		9,338,000		9,315,000									0		0			48,122,000	6,876,000		6,876,000		7,205,000	115,442,000		115,442,000		147,187,000	164,488,000		164,488,000		0	26,266,000
Proceeds from the sale of LNR			240,474,000	0																																																			241,000,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets							11,274,000,000					11,274,000,000			12,953,000,000																						1,471,000,000		1,471,000,000		1,482,000,000
Liabilities							9,562,000,000					9,562,000,000			11,190,000,000																						1,141,000,000		1,141,000,000		1,150,000,000
Noncontrolling interests							72,000,000					72,000,000			44,000,000
Equity							1,640,000,000					1,640,000,000			1,719,000,000																						330,000,000		330,000,000		332,000,000
Total revenue							2,377,000,000			2,552,000,000		10,555,000,000		11,089,000,000																							50,000,000	49,000,000	146,000,000	143,000,000
Net (loss) income attributable to investee							(111,000,000)			(34,000,000)		11,000,000		249,000,000																							14,000,000	19,000,000	41,000,000	57,000,000
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																																					44,032,000		44,032,000
Closing share price (in dollars per share)																																					$ 286.12		$ 286.12
Equity Method Investment Market Value																																					473,262,000		473,262,000
Excess of investee's fair value over carrying amount																																					305,644,000		305,644,000
Excess of investee's carrying amount over equity in net assets						145,809,000					145,809,000																										42,224,000		42,224,000
Sales Of Real Estate																																																							1,053,000,000
Income Loss From Equity Method Investments	(32,756,000)	12,683,000	(45,620,000)	142,187,000		(34,209,000)			(8,585,000)		(69,311,000)		88,696,000			(58,746,000)	(22,074,000)	14,737,000	99,649,000	22,690,000	11,118,000	(10,959,000)	(17,982,000)	(36,056,000)	(10,956,000)	3,778,000	81,667,000	0	0	(78,542,000)	0		1,847,000	2,371,000	5,453,000	7,029,000	5,975,000	8,958,000	17,802,000	24,827,000		4,299,000	7,137,000	12,785,000	19,210,000	1,676,000	1,821,000	5,017,000	5,617,000	0	(323,000)	(979,000)	371,000			0	18,731,000	16,600,000	18,731,000	39,319,000		(1,449,000)	82,000	(2,630,000)	(4,526,000)					38,000	(306,000)	(1,586,000)	(4,282,000)	(1,890,000)	(1,717,000)	(6,480,000)	(9,267,000)				(707,000)	(204,000)	(2,158,000)	99,000				47,000	167,000	415,000	732,000		680,000	256,000	1,054,000	890,000		1,858,000	1,744,000	5,776,000	5,244,000		1,225,000	1,224,000	3,714,000	2,036,000		(2,004,000)	(2,839,000)	(6,346,000)	(7,438,000)		(24,000)	(33,000)	(87,000)	(85,000)		477,000	505,000	1,530,000	1,587,000		376,000	591,000	996,000	1,920,000	(3,111,000)	(3,743,000)	(7,647,000)	(2,218,000)	0	(38,000)	(2,358,000)	(872,000)			165,000	347,000	1,450,000	1,007,000		(1,195,000)	(5,880,000)	(3,540,000)	(7,596,000)		(2,081,000)	1,828,000	(3,199,000)	5,243,000
Acquisitions Of Equity Method Investments [Abstract]
Gain Loss On Disposal Of Discontinued Operations	$ 18,996,000	$ 131,088,000	$ 286,990,000	$ 203,801,000	$ 2,909,000