Investments in Partially Owned Entities (Details) - USD ($)	3 Months Ended					9 Months Ended
	Sep. 30, 2015	Aug. 01, 2015	Sep. 30, 2014	Aug. 02, 2014	Mar. 31, 2014	Sep. 30, 2015	Aug. 01, 2015	Sep. 30, 2014	Aug. 02, 2014	Mar. 31, 2015	Dec. 31, 2014	Nov. 01, 2014
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	$ 1,460,178,000					$ 1,460,178,000					$ 1,240,489,000
Our share of Net (Loss) Income	$ (325,000)		$ (26,034,000)			(8,709,000)		$ (78,676,000)
Impairment Losses						$ 256,000		20,842,000
Toys R Us [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	32.50%					32.50%
Equity in net income					$ 75,196,000
Carrying amount of investments in partially owned entities	$ 0		0			$ 0		0			0
Our share of Net (Loss) Income	46,000		(18,418,000)			2,000,000		(74,162,000)
Non-cash impairment losses					$ 75,196,000	355,953,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets		$ 9,732,000,000					$ 9,732,000,000					$ 11,267,000,000
Liabilities		9,056,000,000					9,056,000,000					10,377,000,000
Noncontrolling interests		85,000,000					85,000,000					82,000,000
Equity		591,000,000					591,000,000					$ 808,000,000
Total revenue		2,293,000,000		$ 2,440,000,000			9,601,000,000		$ 10,186,000,000
Net income (loss) income attributable to investee		$ (108,700,000)		$ (133,000,000)			$ (44,700,000)		$ (244,000,000)
Equity Method Investment, Difference Excess Of Underlying Equity Over Carrying Amount	191,859,000					191,859,000
Toys R Us [Member] | Equity In Net Income After Income Taxes [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income	0		(20,357,000)			0		(4,691,000)
Toys R Us [Member] | Management Fees [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income	46,000		1,939,000			2,000,000		5,725,000
Toys R Us [Member] | Impairment Loss [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income	$ 0		0			$ 0		(75,196,000)
Alexanders Inc [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	32.40%					32.40%
Carrying amount of investments in partially owned entities	$ 131,395,000					$ 131,395,000					131,616,000
Excess of investee's carrying amount over equity in net assets	40,527,000					40,527,000
Our share of Net (Loss) Income	7,544,000		7,192,000			22,558,000		20,471,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	1,457,000,000					1,457,000,000					1,423,000,000
Liabilities	1,112,000,000					1,112,000,000					1,075,000,000
Equity	345,000,000					345,000,000					348,000,000
Total revenue	52,000,000		50,000,000			155,000,000		149,000,000
Net income (loss) income attributable to investee	18,000,000		18,000,000			53,000,000		50,000,000
Alexanders Inc [Member] | Equity In Net Income After Income Taxes [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income	5,716,000		5,552,000			16,757,000		15,583,000
Alexanders Inc [Member] | Management Leasing And Development Fees [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income	1,828,000		1,640,000			5,801,000		4,888,000
India real estate ventures [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	48,114,000					48,114,000					76,752,000
Our share of Net (Loss) Income	(1,704,000)		(262,000)			(18,380,000)		(2,440,000)
Non-cash impairment losses	$ 14,806,000					$ 14,806,000
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 entities	$ 857,282,000					$ 857,282,000					760,749,000
Our share of Net (Loss) Income	$ (2,039,000)		18,000			$ (14,573,000)		(1,387,000)
Urban Edge Properties [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	5.40%					5.40%
Carrying amount of investments in partially owned entities	$ 25,401,000					$ 25,401,000					0
PREIT Associates [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage	8.10%					8.10%				8.10%
Carrying amount of investments in partially owned entities	$ 138,265,000					$ 138,265,000					0
Excess of investee's carrying amount over equity in net assets	65,681,000					65,681,000
Other equity method investments [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	259,721,000					259,721,000					$ 271,372,000
Our share of Net (Loss) Income	$ (4,172,000)		(14,564,000)			$ (314,000)		$ (21,158,000)
Other equity method investments [Member] | Suffolk Downs [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Non-cash impairment losses			10,263,000
Impairment Losses			5,959,000
Asset Impairment Loan Loss Reserve			$ 4,304,000