Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 sqft
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 2,165,365	$ 2,298,367	$ 2,237,583
New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,017,859	786,565	764,044
Square Footage Reclassed				1,000,000
New York Segment [Member] | Alexanders Inc [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	231,402	53,663	49,869
New York Segment [Member] | Hotel Pennsylvania [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	28,455	30,135	23,763
New York Segment [Member] | Retail Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	189,484	163,033	180,225
New York Segment [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	568,518	539,734	510,187
Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	532,412	481,077	497,551
Retail Properties Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	200,526	303,202	302,563
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	172,708	210,022	180,323
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	27,818	93,180	122,240
Merchandise Mart Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	154,538	143,452	84,058
Toys R Us Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	281,289	339,510	334,762
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(21,259)	244,561	254,605
Other Segment [Member] | LNR [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	79,520	47,614	6,116
Other Segment [Member] | Other Properties 555 California Street [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	46,167	44,724	46,782
Other Segment [Member] | Lexington Realty Trust [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	32,595	34,779	41,594
Other Segment [Member] | Lexington Realty Trust [Member] | Net Gains From Stock Issuances [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	28,763	9,760	13,710
Other Segment [Member] | Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	29,266	33,529	30,463
Other Segment [Member] | Alexanders Inc [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,748	7,417	7,556
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	212,152	169,934	125,458
Other Segment [Member] | Corporate General And Administrative Expenses Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(90,567)	(85,922)	(90,343)
Other Segment [Member] | Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	35,397	52,405	65,499
Other Segment [Member] | Derivative Positions in Marketable Equity Securities [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(75,815)	12,984	130,153
Other Segment [Member] | Acquisition Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(17,386)	(5,925)	(36,958)
Other Segment [Member] | Net Gain On Sale Of Condominiums [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,274	5,884	3,149
Other Segment [Member] | Mezzanine Loans Receivable [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	82,744	53,100
Other Segment [Member] | Net Gan Resulting From The Dilution Of Our Investment In Suffolk Downs [Member] | Net Gains From Stock Issuances [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	12,525	0
Other Segment [Member] | Real estate Fund organization costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(3,451)	(5,937)
Other Segment [Member] | Net (loss) gain on early extinguishment of debt [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(10,782)
Other Segment [Member] | Marketable Equity Securities [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(224,937)	0	0
Other Segment [Member] | Partially Owned Entities [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(4,318)	(13,794)	0
Other Segment [Member] | Independence Plaza [Member] | Net Gain On Asset Purchase [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	105,366	0	0
Other Segment [Member] | Verde Realty Operating Partnership [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(4,936)	0	0
Other Segment [Member] | Income before net realized / unrealized gains (Operations) [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,926	4,205	503
Other Segment [Member] | Carried Interest [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,838	736	0
Other Segment [Member] | Unrealized Gain From Mark To Market [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,840	2,999	0
Other Segment [Member] | Realized Gain [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	1,348	0
Other Segment [Member] | Our Total Share Of Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 24,604	$ 9,288	$ 503