Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 506,617	$ 413,263	$ 1,227,982	$ 1,119,625
New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	235,723	210,421	453,260	403,373
Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	84,778	96,312	171,022	196,824
Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	118,406	76,352	440,732	147,095
Merchandise Mart Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	22,448	17,349	37,161	32,649
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	58,656	(6,327)	11,953	109,879
Office Properties [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	158,186	142,077	304,482	277,257
Office Properties [Member] | Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	73,679	85,614	148,948	175,478
Retail [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	57,230	45,577	117,612	90,497
Alexanders Inc [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,213	13,026	20,754	26,397
Hotel Pennsylvania [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,094	9,741	10,412	9,222
Washington Dc Office Excluding Skyline Properties [Member] | Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	66,136	74,953	133,243	153,287
Washington DC Skyline Properties [Member] | Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,543	10,661	15,705	22,191
Washington Dc Residential Property [Member] | Washington DC Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,099	10,698	22,074	21,346
Retail Segment Strip Shopping Centers [Member] | Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	101,529	52,268	204,890	99,176
Retail Segment Strip Shopping Centers [Member] | Litigation Settlement Income [Member] | Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization			59,599
Retail Segment Regional Malls [Member] | Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	16,877	24,084	235,842	47,919
Retail Segment Regional Malls [Member] | Net Gain On Sale Of Real Estate [Member] | Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization			202,275
Gallery At Market Street [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	33,058		33,058
Plant [Member] | Net Gain On Sale Of Real Estate [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	32,169		32,169
LNR [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	11,671	20,443	27,233
Other Properties 555 California Street [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,022	10,377	21,651	20,692
Lexington Realty Trust [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	7,703	6,931	16,921
Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,014	11,523	12,890	20,823
Corporate general and administrative expenses [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(24,831)	(21,812)	(47,587)	(44,129)
Investment income and other, net [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	16,709	15,294	28,045	27,628
Impairment loss on J.C. Penney owned shares [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization			(39,487)
Gain Loss on Sale of J.C. Penney shares [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization			(36,800)
Income from the mark-to-market of J.C. Penney derivative position [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,065	(58,732)	(13,475)	(57,687)
Merchandise Mart Reduction In Force And Severance Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,542)	0	(4,154)	(506)
Merchandise Mart Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5	(6,410)	2,146	56,401
Aquisiiton Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(3,350)	(2,559)	(3,951)	(3,244)
Net Gain On Sale Of Residential Condominiums [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,005	1,274	1,005	1,274
Other Ebitda Attributable To Identifiable Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	61,595	66,618	126,211	130,142
Income before net realized/unrealized gains [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,713)	170	(251)	2,288
Net unrealized gains [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,398	5,284	11,777	6,995
Carried Interest [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,426	2,541	15,609	2,541
Total [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 20,111	$ 7,995	$ 27,135	$ 11,824