Investments in Partially Owned Entities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entitiies	$ 1,499,792,000		$ 1,499,792,000		$ 1,550,422,000
Our share of Net Income (Loss)	642,000	$ (5,641,000)	(3,598,000)	$ (8,384,000)
Other liabilities	$ 480,030,000		$ 480,030,000		426,965,000
7 West 34th Street [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	53.00%		53.00%
Carrying amount of investments in partially owned entitiies					0
Deferred gain	$ 43,813,000		$ 43,813,000
Sale of ownership (percent)	47.00%		47.00%
Other liabilities	$ (43,160,000)		$ (43,160,000)
Alexanders Inc [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	32.40%		32.40%
Equity in net income or earnings	$ 6,812,000	5,447,000	$ 13,749,000	11,041,000
Management, leasing and development fees	1,688,000	1,876,000	3,413,000	3,973,000
Carrying amount of investments in partially owned entitiies	129,795,000		129,795,000		133,568,000
Our share of Net Income (Loss)	8,500,000	7,323,000	17,162,000	15,014,000
India real estate ventures [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entitiies	45,139,000		45,139,000		48,310,000
Our share of Net Income (Loss)	$ (1,934,000)	(16,567,000)	$ (2,620,000)	(16,676,000)
India real estate ventures [Member] | Minimum [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	4.10%		4.10%
India real estate ventures [Member] | Maximum [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	36.50%		36.50%
Partially owned office buildings [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entitiies	$ 811,984,000		$ 811,984,000		909,782,000
Our share of Net Income (Loss)	$ (12,462,000)	(3,238,000)	$ (26,711,000)	(12,534,000)
Urban Edge Properties [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	5.40%		5.40%
Equity in net income or earnings	$ 1,071,000	404,000	$ 1,947,000	404,000
Management, leasing and development fees	209,000	500,000	418,000	1,084,000
Carrying amount of investments in partially owned entitiies	23,868,000		23,868,000		25,351,000
Our share of Net Income (Loss)	$ 1,280,000	904,000	$ 2,365,000	1,488,000
PREIT Associates [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	8.00%		8.00%
Carrying amount of investments in partially owned entitiies	$ 125,822,000		$ 125,822,000		133,375,000
Our share of Net Income (Loss)	(527,000)	(364,000)	(4,815,000)	(364,000)
Other equity method investments [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entitiies	363,184,000		363,184,000		$ 300,036,000
Our share of Net Income (Loss)	$ 5,785,000	$ 6,301,000	$ 11,021,000	$ 4,688,000