Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 2,242,455	$ 2,182,355	$ 1,672,004
Net Gains From Stock Issuances [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,760	13,710
Income before net realized / unrealized gains (Operations) [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,205	503	0
Carried Interest [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	736	0	0
Unrealized Gain From Mark To Market [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,999	0	0
Realized Gain [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,348	0	0
Our Total Share Of Real Estate Fund [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,288	503	0
Alexanders Inc [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	61,080	57,425	81,703
California Street 555 [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	44,724	46,782	44,757
Lexington Realty Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	44,539	55,304	50,024
Hotel Pennsylvania [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	30,135	23,763	15,108
LNR [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	47,614	6,116	0
Other Investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	33,529	30,463	11,070
Other EBITDA attributable to identifiable investments [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	270,909	220,356	202,662
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	(85,922)	(90,343)	(79,843)
Investment Income And Other Excluding Mark To Market Of Deferred Compensation Plan Assets And Offsetting Liability [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	52,405	65,499	78,593
Net (income) loss attributable to noncontrolling interests, including unit distributions [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(55,912)	(55,228)	(25,120)
Derivative Positions in Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,984	130,153	0
Real estate Fund organization costs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(3,451)	(5,937)	0
Net (loss) gain on early extinguishment of debt [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(10,782)	(26,684)
Mezzanine Loans Receivable [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	82,744	53,100	(190,738)
Investment In Lexington Reality Trust [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(19,121)
Marketable Equity Securities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(3,361)
Wholly Owned Entities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	(30,013)	(39,299)
Partially Owned Entities [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(13,794)	0	(17,820)
Other Segment [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	272,447	273,009	(140,285)
Write Off Of Unamortized Costs From Voluntary Surrender Of Equity Awards [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(20,202)
Net Gain On Sale Of Condominiums [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,884	3,149	648
Acquisition Related Costs
Earnings Before Interest, Taxes, Depreciation, and Amortization	(5,925)	(6,945)	0
Net Gan Resulting From The Dilution Of Our Investment In Suffolk Downs [Member]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 12,525	$ 0	$ 0