Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 687,217	$ 864,187
Alexander's Inc. [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,889	1,887
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,315	10,965
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,218	10,541
Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	1,452
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	15,562	9,390
Other Investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,300	8,201
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	48,224	40,251
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	(22,317)	(21,355)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,445	13,083
Net Income Loss Attributable To Noncontrolling Interest Operating Partnerships And Preferred Unit Holders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(19,145)	(31,808)
Income (Loss) from the Mark-to-Market of Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,045	17,163
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(3,048)
Mezzanine Loans Receivable [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	82,744
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	19,456	104,725
Net gain on sale of condominiums [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	4,586
Acquisition related costs
Earnings Before Interest, Taxes, Depreciation, and Amortization	(685)	(230)
Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,829	1,154
Real Estate Fund [Member] | Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,118	980
Real Estate Fund [Member] | Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 1,711	$ 174