Segment Information (Parentheticals) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 476,690	$ 444,581	$ 1,340,877	$ 1,097,175
Alexanders Inc [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	15,821	14,260	30,989	28,659
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,423	11,136	21,388	22,624
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	17,313	11,435	29,306	29,283
Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,308		9,760	5,998
Hotel Pennsylvania [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,677	6,616	8,609	6,169
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,410	0	22,800	0
LNR [Member] | Net Gains From Asset Sales [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,020		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	11,735	8,469	19,936	18,615
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	84,335	51,916	141,138	105,350
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	(20,024)	(20,642)	(41,379)	(39,956)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,954	14,554	26,330	26,068
Net Income Loss Attributable To Noncontrolling Interest Operating Partnerships And Preferred Unit Holders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(8,731)	(4,124)	(40,539)	(21,903)
Income (Loss) from the Mark-to-Market of Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(6,762)	0	10,401	0
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(403)	(2,656)	(3,451)	(2,730)
Mezzanine Loans Receivable [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(6,900)	82,744	(6,900)
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	58,178	29,868	176,116	60,076
Net gain on sale of condominiums [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	722	4,586	3,149
Acquisition costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,191)	(1,930)	(3,714)	(1,930)
Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,956	0	8,110	0
Real Estate Fund [Member] | Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	827	0	1,807	0
Real Estate Fund [Member] | Carried Interest [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,140	0	2,140	0
Real Estate Fund [Member] | Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,218	0	3,392	0
Real Estate Fund [Member] | Realized Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 771	$ 0	$ 771	$ 0