Segment Information (Parentheticals) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 1,809,535	$ 2,229,471	$ 1,993,880
Carrying amount of investments in partially owned entitiies	1,550,422	1,240,489	1,159,803
Non cash impairment losses		(85,459)	(281,098)
Mark-to-market of investments in our deferred compensation plan	111	11,557	10,636
Corporate general and administrative expenses in 2016 due to accelerated compensation expenses	2,940
Corporate general and administrative expenses after 2016 due to accelerated compensation expenses	3,277
Out Performance Plan [Member]
Segment Information [Line Items]
Additional Expense From Acceleration Of Vesting	$ 6,217
Revised Age Limit For Awards Vesting Criteria	65 years
Our share of Toys "R" Us [Member]
Segment Information [Line Items]
Carrying amount of investments in partially owned entitiies				$ 0
Non cash impairment losses		75,196	240,757
New York Segment [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 1,250,819	1,439,189	1,079,682
Carrying amount of investments in partially owned entitiies	1,195,122	1,036,130	904,278
New York Segment [Member] | Net Gain On Sale Of Real Estate [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	142,693	440,537	127,512
New York Segment [Member] | Alexanders Inc [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	42,858	41,746	42,210
New York Segment [Member] | Hotel Pennsylvania [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	23,044	30,753	30,723
New York Segment [Member] | Residential Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	22,266	21,907	20,420
New York Segment [Member] | Retail Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	358,379	281,428	246,808
New York Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	661,579	622,818	612,009
Washington DC Segment [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	430,470	335,590	341,223
Carrying amount of investments in partially owned entitiies	100,511	102,635	100,543
Washington DC Segment [Member] | Washington Dc Office Excluding Skyline Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	264,864	266,859	268,373
Washington DC Segment [Member] | Net gain on sale of 1750 Pennsylvania Avenue [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	102,404	0	0
Washington DC Segment [Member] | Skyline Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	24,224	27,150	29,499
Washington DC Segment [Member] | Residential Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	38,978	41,581	43,351
Washington DC Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	391,492	294,009	297,872
Other Segment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	128,246	454,692	572,975
Carrying amount of investments in partially owned entitiies	254,789	101,724	154,982
Other Segment [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	33,964	70,306	49,471
Other Segment [Member] | The Mart and trade shows [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	79,159	79,636	74,270
Other Segment [Member] | 555 California Street [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	49,975	48,844	42,667
Other Segment [Member] | India real estate ventures [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,933	6,434	5,841
Other Segment [Member] | Our share of Toys "R" Us [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,500	103,632	(12,081)
Other Segment [Member] | Other Investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	38,141	16,896	45,856
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(106,416)	(94,929)	(94,904)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	26,385	31,665	46,525
Other Segment [Member] | Our share of impairment loss on India real estate ventures [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(14,806)	(5,771)	0
Other Segment [Member] | Gains on sale of partially owned entities and other [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	37,666	13,000	0
Other Segment [Member] | UE and residual retail properties discontinued operations [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	28,314	245,679	541,516
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(12,511)	(16,392)	(24,857)
Other Segment [Member] | Net gain on sale of marketable securities, land parcels and residential condominiums [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,724	13,568	56,868
Other Segment [Member] | Net income attributable to noncontrolling interests in the Operating Partnership [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(43,231)	(47,613)	(24,817)
Other Segment [Member] | Impairment loss and loan loss reserve on investment in Suffolk Downs [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,551)	(10,263)	0
Other Segment [Member] | Losses from the disposition of investment in J.C. Penney [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(127,888)
Other Segment [Member] | Severance costs (primarily reduction in force at the Mart) [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(5,492)
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	207,672	325,748	206,024
Other Segment [Member] | Income before net realized/unrealized gains [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,611	8,056	7,752
Other Segment [Member] | Net unrealized/realized gains on investments [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	14,657	37,535	23,489
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 10,696	$ 24,715	$ 18,230