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	$ 515,463	$ 474,278	$ 1,266,607	$ 1,393,514
Carrying amount of investments in partially owned entitiies	1,460,178		1,460,178		$ 1,240,489
Impairment losses			256	20,842
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	50,385	139,976	107,705	405,104
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] | Net income attributable to noncontrolling interests in the Operating Partnership [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(12,704)	(7,988)	(28,189)	(16,552)
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