Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 1,993,880	$ 2,120,102	$ 2,242,455
New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,079,682	1,017,859	786,565
New York Segment [Member] | Alexanders Inc [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	42,210	231,402	53,663
New York Segment [Member] | Alexanders Inc [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		179,934
New York Segment [Member] | Hotel Pennsylvania [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	30,723	28,455	30,135
New York Segment [Member] | Retail Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	246,808	189,484	163,033
New York Segment [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	759,941	568,518	539,734
New York Segment [Member] | Office Properties [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	127,512
Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	341,223	532,412	481,077
Washington DC Segment [Member] | Washington Dc Office Excluding Skyline Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	268,373	449,448	385,285
Washington DC Segment [Member] | Washington Dc Office Excluding Skyline Properties [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		163,367
Washington DC Segment [Member] | Washington DC Skyline Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	29,499	40,037	56,148
Washington DC Segment [Member] | Washington Dc Residential Property [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	43,351	42,927	39,644
Washington DC Segment [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	297,872	489,485	441,433
Retail Properties Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	541,017	200,526	303,202
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	285,612	172,708	210,022
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	19,000	33,775
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	81,806	15,821
Retail Properties Segment [Member] | Retail Segment Strip Shopping Centers [Member] | Litigation Settlement Income [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	59,599
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	255,405	27,818	93,180
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,443
Retail Properties Segment [Member] | Retail Segment Regional Malls [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	202,275	70,100
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	44,039	88,016	332,101
Other Segment [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	49,471	24,604	9,288
Other Segment [Member] | LNR [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	20,443	75,202	47,614
Other Segment [Member] | Merchandise Mart Other [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	74,270	62,470	50,406
Other Segment [Member] | Other Properties 555 California Street [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	42,667	46,167	44,724
Other Segment [Member] | India real estate ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,841	3,654	7,037
Other Segment [Member] | Lexington Realty Trust [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,931	32,595	34,779
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	0	28,763	9,760
Other Segment [Member] | Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	18,981	25,612	26,092
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	218,604	270,304	219,940
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	(94,904)	(89,082)	(85,922)
Other Segment [Member] | Marketable Equity Securities [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(39,487)	(224,937)	0
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	46,525	45,563	55,202
Other Segment [Member] | Net Gain On Sale Of Marketable Securities Land Parcels And Residential Condominimums [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	56,868	4,856	10,904
Other Segment [Member] | Loss On Sale Of J C Penney Common Shares [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(54,914)	0	0
Other Segment [Member] | Derivative Positions in Marketable Equity Securities [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(33,487)	(75,815)	12,984
Other Segment [Member] | Acquisition Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(24,857)	(17,386)	(5,925)
Other Segment [Member] | Mezzanine Loans Receivable [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	82,744
Other Segment [Member] | Severance Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(5,492)	(3,005)	(4,226)
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	0	12,525
Other Segment [Member] | Real estate Fund organization costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(3,451)
Other Segment [Member] | Merchandise Mart Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	93,588	97,272
Other Segment [Member] | Partially Owned Entities [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(13,794)
Other Segment [Member] | Independence Plaza [Member] | Net Gain On Asset Purchase [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	105,366	0
Other Segment [Member] | Verde Realty Operating Partnership [Member] | Impairment Loss [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(4,936)	0
Other Segment [Member] | Net Income Attributable To Operating Partnership [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(23,659)	(35,327)	(41,059)
Other Segment [Member] | Preferred Unit Distributions Of The Operating Partnership [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,158)	(9,936)	(14,853)
Other Segment [Member] | Income before net realized / unrealized gains (Operations) [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,676	4,926	4,205
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	24,306	5,838	736
Other Segment [Member] | Unrealized Gain From Mark To Market [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	21,443	13,840	2,999
Other Segment [Member] | Realized Gain [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 2,046	$ 0	$ 1,348