Investments in Partially Owned Entities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Nov. 01, 2014	Dec. 31, 2013	Nov. 02, 2013	Mar. 31, 2015	Sep. 30, 2014
Equity Method Investments And Income From Equity Method Investments [Abstract]
Non cash impairment losses			$ (85,459)		$ (281,098)
Carrying amount of investments in partially owned entities		$ 1,550,422	1,240,489		1,159,803
Our share of Net (Loss) Income		(12,630)	(59,861)		(340,882)
Non-cash impairment losses		0	0		72,974
Impairment Losses		256	26,518		37,170
Condensed Financial Information For Equity Method Investments [Abstract]
Assets		25,526,000	21,389,000
Liabilities		21,162,000	17,986,000
Noncontrolling interests		146,000	104,000
Equity		4,218,000	3,299,000
Total revenue		13,423,000	13,620,000		14,092,000
Net (loss) income attributable to investee		$ (224,000)	(434,000)		(368,000)
Toys R Us [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		32.50%
Equity in net income (loss)		$ 0	(4,691)		(128,919)
Non cash impairment losses	$ 75,196	0	(75,196)		(240,757)
Management fees		2,500	6,331		7,299
Carrying amount of investments in partially owned entities		0	0					$ 0
Our share of Net (Loss) Income	$ 75,196	$ 2,500	(73,556)		(362,377)
Condensed Financial Information For Equity Method Investments [Abstract]
Assets				$ 11,267,000
Liabilities				10,377,000
Noncontrolling interests				82,000
Equity				808,000
Total revenue				12,645,000		$ 13,046,000
Net (loss) income attributable to investee				$ (343,000)		$ (396,000)
Alexanders Inc [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		32.40%
Equity in net income (loss)		$ 24,209	21,287		17,721
Management, leasing and development fees		6,869	8,722		6,681
Carrying amount of investments in partially owned entities		133,568	131,616
Excess of investee's carrying amount over equity in net assets		40,340
Our share of Net (Loss) Income		31,078	30,009		24,402
India real estate ventures [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Non cash impairment losses		14,806	5,771
Carrying amount of investments in partially owned entities		48,310	76,752
Our share of Net (Loss) Income		$ (18,746)	(8,309)		(3,533)
India real estate ventures [Member] | Minimum [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		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%
India real estate ventures [Member] | Tcg Urban Infrastructure Holdings [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		25.00%
Partially owned office buildings [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities		$ 909,782	760,749
Our share of Net (Loss) Income		(23,556)	93		(4,212)
Partially owned office buildings [Member] | Joint Venture [Member]
Condensed Financial Information For Equity Method Investments [Abstract]
Equity		0
Partially owned office buildings [Member] | 666 Fifth Avenue [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income		(39,600)
Partially owned office buildings [Member] | 650 Madison Avenue [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Write off of below market lease liability		$ 12,800
Partially owned office buildings [Member] | West 57th Street [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of accelerated depreciation			14,500
Urban Edge Properties [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		5.40%
Equity in net income (loss)		$ 2,430	0		0
Management fees		1,964	0		0
Carrying amount of investments in partially owned entities		25,351	0
Our share of Net (Loss) Income		$ 4,394	0		0
PREIT Associates [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Equity method ownership percentage		8.10%					8.10%
Carrying amount of investments in partially owned entities		$ 133,375	0
Excess of investee's carrying amount over equity in net assets		65,404
Our share of Net (Loss) Income		(7,450)	0		0
LNR [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income		0	0		18,731
Non-cash impairment losses					27,231
Condensed Financial Information For Equity Method Investments [Abstract]
Net (loss) income attributable to investee					18,731
LNR [Member] | Net gain on sale [Member]
Condensed Financial Information For Equity Method Investments [Abstract]
Net (loss) income attributable to investee					3,776
LNR [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					42,186
Lexington Realty Trust [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income		0	0		(979)
Other equity method investments [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Carrying amount of investments in partially owned entities		300,036	271,372
Our share of Net (Loss) Income		(850)	(8,098)		(12,914)
Other equity method investments [Member] | Suffolk Downs [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Non-cash impairment losses			10,263
Impairment Losses			5,959
Asset Impairment Loan Loss Reserve			4,304
All Equity Method Investees Excluding Toys [Member]
Equity Method Investments And Income From Equity Method Investments [Abstract]
Our share of Net (Loss) Income		$ (12,630)	$ (59,861)		$ (340,882)