Investments in Partially Owned Entities (Details) - Real Estate And Accumulated Depreciation Name Of Property [Domain] - USD ($)	3 Months Ended					6 Months Ended
	Jun. 30, 2015	May. 02, 2015	Jun. 30, 2014	May. 03, 2014	Mar. 31, 2014	Jun. 30, 2015	May. 02, 2015	Jun. 30, 2014	May. 03, 2014	Mar. 31, 2015	Dec. 31, 2014	Nov. 01, 2014	Sep. 30, 2014
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	$ 1,477,090,000					$ 1,477,090,000					$ 1,246,496,000
Income (Loss) From Equity Method Investments	$ (5,231,000)		$ (53,742,000)			$ (7,636,000)		$ (51,763,000)
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
Income (Loss) From Equity Method Investments	500,000		(57,591,000)			1,954,000		(55,744,000)
Non-cash impairment losses					$ 75,196,000	355,953,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets		$ 9,772,000,000					$ 9,772,000,000					$ 11,267,000,000
Liabilities		8,965,000,000					8,965,000,000					10,377,000,000
Noncontrolling interests		85,000,000					85,000,000					82,000,000
Equity		722,000,000					722,000,000					$ 808,000,000
Total revenue		2,325,000,000		$ 2,479,000,000			7,308,000,000		$ 7,746,000,000
Net (loss) income attributable to investee		$ (129,700,000)		$ (194,000,000)			$ 64,000,000		$ (111,000,000)
Equity Method Investment, Difference Excess Of Underlying Equity Over Carrying Amount	234,553,000					234,553,000
Toys R Us [Member] | Equity In Net Income After Income Taxes [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	0		(59,530,000)			0		15,666,000
Toys R Us [Member] | Management Fees [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	500,000		1,939,000			1,954,000		3,786,000
Toys R Us [Member] | Impairment Loss [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	$ 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	$ 130,639,000					$ 130,639,000					131,616,000
Excess of investee's carrying amount over equity in net assets	40,690,000					40,690,000
Income (Loss) From Equity Method Investments	7,323,000		6,894,000			15,014,000		13,279,000
Condensed Financial Information For Equity Method Investments [Abstract]
Assets	1,418,000,000					1,418,000,000					1,423,000,000
Liabilities	1,074,000,000					1,074,000,000					1,075,000,000
Equity	344,000,000					344,000,000					348,000,000
Total revenue	51,000,000		50,000,000			103,000,000		99,000,000
Net (loss) income attributable to investee	17,000,000		17,000,000			35,000,000		32,000,000
Alexanders Inc [Member] | Equity In Net Income After Income Taxes [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	5,447,000		5,272,000			11,041,000		10,031,000
Alexanders Inc [Member] | Management Leasing And Development Fees [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	1,876,000		1,622,000			3,973,000		3,248,000
India real estate ventures [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	50,542,000					50,542,000					76,752,000
Income (Loss) From Equity Method Investments	(16,567,000)		(2,041,000)			(16,676,000)		(2,178,000)
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	$ 859,544,000					$ 859,544,000					760,749,000
Income (Loss) From Equity Method Investments	$ (3,238,000)		990,000			$ (12,534,000)		(1,405,000)
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	$ 25,610,000					$ 25,610,000					0
Income (Loss) From Equity Method Investments	904,000		0			1,488,000		0
Urban Edge Properties [Member] | Equity In Net Income After Income Taxes [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	404,000		0			404,000		0
Urban Edge Properties [Member] | Management Leasing And Development Fees [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Income (Loss) From Equity Method Investments	$ 500,000		0			$ 1,084,000		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	$ 143,031,000					$ 143,031,000					0
Other equity method investments [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities	267,724,000					267,724,000					$ 277,379,000
Income (Loss) From Equity Method Investments	$ 5,847,000		$ (1,994,000)			$ 3,118,000		$ (5,715,000)