Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 605,990	$ 437,670	$ 1,701,260	$ 1,778,547
Alexander's Inc. [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,821	1,758	5,617	5,545
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,714	11,220	31,406	32,608
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,859	8,424	24,780	27,970
Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	0	9,760
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	18,773	15,769	46,006	38,569
Other Investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,121	10,173	24,954	30,352
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	49,189	47,264	139,692	139,287
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	(22,811)	(21,585)	(66,940)	(62,964)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,033	12,541	28,865	37,284
Net Income Loss Attributable To Noncontrolling Interest Operating Partnerships And Preferred Unit Holders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(16,240)	(6,825)	(40,595)	(47,364)
Income (Loss) from the Mark-to-Market of Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,344	(37,537)	(53,343)	(27,136)
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	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	0	1,298	1,274	5,884
Verde Realty impairment loss [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(4,936)	0	(4,936)	0
Acquisition related costs
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,070)	(695)	(4,314)	(2,822)
Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	722	1,678	12,546	9,788
Real Estate Fund [Member] | Operations [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,874	743	4,162	2,550
Real Estate Fund [Member] | Carried Interest [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,541)	(475)	0	1,665
Real Estate Fund [Member] | Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,389	1,410	8,384	4,802
Real Estate Fund [Member] | Realized Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	0	771
New York Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	206,663	203,283	610,036	579,442
New York Segment [Member] | Ny Segment Ny Office [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	139,894	137,295	419,054	399,182
New York Segment [Member] | Ny Segment Alexanders [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,080	12,830	39,477	40,032
New York Segment [Member] | NY Segment Hotel Pennsylvania [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,524	9,773	16,746	18,382
New York Segment [Member] | NY Segment Retail [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	46,165	43,109	135,399	121,136
Washington DC Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	217,567	106,607	414,391	374,937
Retail Properties Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	73,505	68,339	220,600	208,496
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	42,468	41,282	125,072	120,887
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	15,877	15,769	47,277	46,621
Merchandise Mart Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	44,942	15,448	133,486	145,130
Toys R Us Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	47,983	47,872	310,437	334,116
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 15,330	$ (3,879)	$ 12,310	$ 136,426