Segment Information (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 706,646	$ 687,217
New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	217,537	192,952
Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	322,326	70,743
Merchandise Mart Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	14,713	15,300
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(61,422)	97,061
Retail Segment Strip Shopping Centers [Member] | Total Retail properties [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	103,361	46,908
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	218,965	23,835
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
Office Properties [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	146,296	135,180
Retail [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	60,382	44,920
Hotel Pennsylvania [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	318	(519)
LNR [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	20,443	15,562
Other Properties 555 California Street [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,629	10,315
Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,807	18,518
Corporate general and administrative expenses [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(22,756)	(22,317)
Investment income and other, net [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,336	12,334
Alexanders Inc [Member] | New York Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,541	13,371
Impairment loss on J.C. Penney owned shares [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(39,487)	0
Gain Loss on Sale of J.C. Penney shares [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(36,800)	0
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	(22,540)	1,045
Merchandise Mart Reduction In Force And Severance Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,612)	(506)
Merchandise Mart Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,141	7,900
Aquisiiton Costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(601)	(685)
Net Gain On Sale Of 350 West Mart [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	54,911
Net Income Attributable To Operating Partnership [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(13,933)	(15,271)
Preferred Unit Distributions Of The Operating Partnership [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(786)	(3,874)
Other Ebitda Attributable To Identifiable Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	64,616	63,524
Income before net realized/unrealized gains [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,462	2,118
Net unrealized gains [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,379	1,711
Carried Interest [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,183	0
Total [Member] | Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 7,024	$ 3,829