Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 408,053	$ 476,690	$ 1,095,270	$ 1,340,877
Alexander's Inc. [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,907	1,900	3,796	3,787
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,377	10,423	20,692	21,388
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,703	9,005	16,921	19,546
Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	8,308	0	9,760
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,671	13,410	27,233	22,800
Other Investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,523	11,735	20,823	19,936
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	49,269	51,529	97,493	91,780
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	(21,812)	(20,024)	(44,129)	(41,379)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,387	11,660	23,832	24,743
Net Income Loss Attributable To Noncontrolling Interest Operating Partnerships And Preferred Unit Holders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(5,210)	(8,731)	(24,355)	(40,539)
Income (Loss) from the Mark-to-Market of Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(58,732)	(6,762)	(57,687)	10,401
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(403)	0	(3,451)
Mezzanine Loans Receivable [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	0	82,744
Net gain on sale of condominiums [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,274	0	1,274	4,586
Acquisition related costs
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,559)	(1,897)	(3,244)	(2,127)
Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,995	6,956	11,824	8,110
Real Estate Fund [Member] | Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	170	827	2,288	1,807
Real Estate Fund [Member] | Carried Interest [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,541	2,140	2,541	2,140
Real Estate Fund [Member] | Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,284	3,218	6,995	3,392
Real Estate Fund [Member] | Realized Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	771	0	771
New York Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	210,421	207,610	403,373	376,159
New York Segment [Member] | Ny Segment Ny Office [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	142,573	137,630	278,520	262,321
New York Segment [Member] | Ny Segment Alexanders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,026	13,921	26,397	27,202
New York Segment [Member] | NY Segment Hotel Pennsylvania [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,741	8,677	9,222	8,609
New York Segment [Member] | NY Segment Retail [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	45,081	47,382	89,234	78,027
Washington DC Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	96,312	111,517	196,824	268,330
Retail Properties Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	76,352	68,511	147,095	140,157
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	52,268	45,622	99,176	95,782
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	24,084	22,889	47,919	44,375
Retail Properties Segment [Member] | Retail Segment Continuing Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	41,438	39,564	82,604	79,605
Merchandise Mart Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,939	23,998	88,544	129,682
Toys R Us Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	36,505	29,474	262,454	286,244
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ (22,476)	$ 35,580	$ (3,020)	$ 140,305