Segment Information (Parentheticals) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 437,670	$ 468,681	$ 1,778,547	$ 1,565,856
Alexanders Inc [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	14,588	13,288	45,577	41,947
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,220	11,797	32,608	34,421
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,424	8,092	37,730	37,375
Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0		9,760	5,998
Hotel Pennsylvania [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,773	8,080	18,382	14,249
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	15,769	0	38,569	0
LNR [Member] | Net Gains From Asset Sales [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0		6,020
LNR [Member] | Tax Settlement Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization			8,977
Other Investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,075	4,004	30,011	22,619
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	71,527	44,942	212,665	150,292
Corporate General And Administrative Expenses Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(21,585)	(20,712)	(62,964)	(60,668)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,530	15,808	38,860	41,876
Net Income Loss Attributable To Noncontrolling Interest Operating Partnerships And Preferred Unit Holders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(6,825)	(11,584)	(47,364)	(33,487)
Income (Loss) from the Mark-to-Market of Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(37,537)	32,249	(27,136)	32,249
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(3,207)	(3,451)	(5,937)
Net (loss) gain on early extinguishment of debt [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization		(724)		(1,796)
Mezzanine Loans Receivable [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	82,744	(6,900)
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	18,724	55,851	194,840	115,927
Net gain on sale of condominiums [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,298	0	5,884	3,149
Acquisition related costs
Earnings Before Interest, Taxes, Depreciation, and Amortization	(684)	(921)	(4,398)	(2,851)
Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,678	(319)	9,788	(319)
Real Estate Fund [Member] | Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	743	(319)	2,550	(319)
Real Estate Fund [Member] | Carried Interest [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(475)	0	1,665	0
Real Estate Fund [Member] | Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,410	0	4,802	0
Real Estate Fund [Member] | Realized Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 0	$ 0	$ 771	$ 0