Segment Information (Parentheticals) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 528,167	$ 482,266	$ 1,294,796	$ 1,410,066
Carrying amount of investments in partially owned entitiies	1,460,178		1,460,178		$ 1,240,489
Mark-to-market of investments in our deferred compensation plan	(2,577)	1,352	(327)	8,132
Transaction Costs Spin Off	0	5,828	22,972	9,343
Out Performance Plan [Member]
Segment Reporting Information [Line Items]
Additional Expense From Acceleration Of Vesting			$ 7,084
Revised Age Limit For Awards Vesting Criteria			65 years
Our share of Toys "R" Us [Member]
Segment Reporting Information [Line Items]
Carrying amount of investments in partially owned entitiies		0		0
Non-cash impairment losses				75,196
Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Mark-to-market of investments in our deferred compensation plan	(2,577)	1,352	$ (327)	8,132
Corporate General And Administrative Expenses For Remainder Of 2015			867
Corporate General And Administrative Expenses After 2015			6,217
Retail Segment Strip Shopping Centers [Member]
Segment Reporting Information [Line Items]
Transaction Costs Spin Off		5,828	22,972	9,343
New York Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	282,390	250,583	807,323	735,710
New York Segment [Member] | Alexanders Inc [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,502	10,387	31,150	31,088
New York Segment [Member] | Hotel Pennsylvania [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,621	9,301	14,351	18,873
New York Segment [Member] | Retail Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	97,604	71,327	265,060	205,469
New York Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	166,663	159,568	496,762	480,280
Washington DC Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	182,688	83,719	351,579	252,700
Washington DC Segment [Member] | Washington Dc Office Excluding Skyline Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	64,733	65,904	200,631	200,218
Washington DC Segment [Member] | Washington DC Office 1750 Pennsylvania Avenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	102,404	0	102,404	0
Washington DC Segment [Member] | Washington DC Skyline Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,998	7,698	19,037	21,270
Washington DC Segment [Member] | Washington Dc Residential Property [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,553	10,117	29,507	31,212
Washington DC Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	173,135	73,602	322,072	221,488
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	63,089	147,964	135,894	421,656
Other Segment [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,623	15,572	22,669	61,201
Other Segment [Member] | Merchandise Mart Other [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	19,044	19,497	62,229	61,038
Other Segment [Member] | Other Properties 555 California Street [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13,005	11,994	38,237	35,566
Other Segment [Member] | India real estate ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	13	2,651	2,229	4,574
Other Segment [Member] | Our share of Toys "R" Us [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	46	12,440	2,000	103,026
Other Segment [Member] | Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,558	4,372	25,787	13,594
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(22,341)	(22,948)	(82,043)	(71,952)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,952	6,659	21,275	22,764
Other Segment [Member] | Our share of impairment losses on India real estate ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(14,806)	0
Other Segment [Member] | Gains on sale of partially owned entities and other [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	33,153	0	37,666	0
Other Segment [Member] | UE and residual retail properties discontinued operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,516	106,602	26,313	192,532
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,518)	(1,277)	(7,560)	(3,629)
Other Segment [Member] | Net gain on sale of residential condominiums and a land parcel
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	633	2,665	2,493	13,205
Other Segment [Member] | Suffolk Downs impairment loss and loan reserve [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(595)	(10,263)	(595)	(10,263)
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	45,289	66,526	153,151	278,999
Other Segment [Member] | Income before net realized/unrealized gains [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,594	2,051	6,879	6,668
Other Segment [Member] | Net unrealized/realized (losses) gains on investments [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(922)	5,083	9,542	32,890
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ (49)	$ 8,438	$ 6,248	$ 21,643