Segment Information (Notes to preceding tabular information) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Information [Line Items]
Mark-to-market income (loss) of investments in our deferred compensation plan	$ 204	$ (2,577)	$ 2,625	$ (327)
Transaction Costs Spin Off	0	0	0	22,972
Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	398,007	528,167	1,162,125	1,294,796
Corporate General And Administrative Expenses
Segment Information [Line Items]
Mark-to-market income (loss) of investments in our deferred compensation plan	204	(2,577)	2,625	(327)
New York Segment | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	276,893	282,390	977,517	807,323
New York Segment | Office | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	159,937	161,168	475,726	480,508
New York Segment | Office | Properties Sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		5,151	2,935	16,954
New York Segment | Office | Excluding properties sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		156,017	472,791	463,554
New York Segment | Retail | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	95,274	97,604	284,212	265,060
New York Segment | Retail | Properties Sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		524	185	1,597
New York Segment | Retail | Excluding properties sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		97,080	284,027	263,463
New York Segment | Residential | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,214	5,495	18,901	16,254
New York Segment | Alexanders Inc | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,506	10,502	34,880	31,150
New York Segment | Hotel Pennsylvania | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,962	7,621	4,287	14,351
New York Segment | Net gain on sale of 7 west 34th street | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	159,511	0
New York Segment | EBITDA before Gain on Sale | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	276,893	282,390	818,006	807,323
Washington DC Segment | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	82,765	182,688	77,834	351,579
Washington DC Segment | Office | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	71,295	172,281	45,123	321,198
Washington DC Segment | Residential | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,470	10,407	32,711	30,381
Washington DC Segment | Skyline Properties | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,222	5,998	14,177	19,037
Washington DC Segment | Skyline Properties | Impairment Loss | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(160,700)	0
Washington DC Segment | Office, excluding the Skyline properties | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	67,073	63,879	191,646	199,757
Washington DC Segment | Office, excluding the Skyline properties | Properties Sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		1,601		5,591
Washington DC Segment | Office, excluding the Skyline properties | Excluding properties sold and other | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		62,278		194,166
Washington DC Segment | 1750 Pennsylvania Avenue | Net Gain On Sale Of Real Estate | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	102,404	0	102,404
Other Segment | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	38,349	63,089	106,774	135,894
Other Segment | Real estate fund investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	807	1,623	13,662	22,669
Other Segment | The Mart and trade shows | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	21,696	19,044	70,689	62,229
Other Segment | 555 California Street | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,405	13,005	35,137	38,237
Other Segment | India real estate ventures | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	836	13	2,585	2,229
Other Segment | Other Investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	19,092	11,009	46,180	31,705
Other Segment | Corporate General And Administrative Expenses | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(21,519)	(22,341)	(76,364)	(82,043)
Mark-to-market income (loss) of investments in our deferred compensation plan	204	(2,577)	2,625	(327)
Additional Expense From Acceleration Of Vesting				4,542
Other Segment | Investment Income and other, net | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,871	5,952	19,317	21,275
Other Segment | Our share of impairment loss on India real estate ventures | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	0	(14,806)
Other Segment | Acquisition and transaction related costs | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(3,808)	(1,518)	(11,319)	(7,560)
Other Segment | Net gain on sale of residential condominiums | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	633	714	2,493
Other Segment | Net gain on sale of Monmouth Mall | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	33,153	0	33,153
Other Segment | Other EBITDA attributable to identifiable investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	53,836	44,694	168,253	157,069
Other Segment | UE and residual retail properties discontinued operations | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,969	2,516	6,173	26,313
Transaction Costs Spin Off				22,972
Other Segment | Income before net realized/unrealized gains | Real estate fund investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,552	2,594	6,309	6,879
Other Segment | Net unrealized/realized (losses) gains on investments | Real estate fund investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,118)	(922)	3,333	9,542
Other Segment | Carried Interest | Real estate fund investments | Operating Segments
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 373	$ (49)	$ 4,020	$ 6,248