Segment Information (Parentheticals) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 561,212	$ 386,879	$ 764,118	$ 766,629
Mark-to-market income (loss) of investments in our deferred compensation plan	4,359	(609)	2,421	2,250
Additional Expense From Acceleration Of Vesting				4,542
Transaction Costs Spin Off	0	327	0	22,972
New York Segment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	440,125	275,988	700,624	524,933
New York Segment [Member] | Alexanders Inc [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,805	10,241	23,374	20,648
New York Segment [Member] | Hotel Pennsylvania [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,797	8,856	325	6,730
New York Segment [Member] | Residential Properties [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,337	5,709	12,687	10,759
New York Segment [Member] | Retail Properties [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	95,615	86,151	188,938	167,456
New York Segment [Member] | Office [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	163,060	165,031	315,789	319,340
New York Segment [Member] | 20 Broad Street [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		3,304		6,844
New York Segment [Member] | 20 Broad Street [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		161,727
New York Segment [Member] | New York Segment Excluding 20 Broad Street [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization				312,496
New York Segment [Member] | Net gain on sale of 7 west 34th street
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	159,511	0	159,511	0
New York Segment [Member] | Total
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	280,614	275,988	541,113	524,933
Washington DC Segment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	79,454	85,634	(4,931)	168,891
Washington DC Segment [Member] | Washington Dc Office Excluding Skyline Properties [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	63,757	68,509	124,573	135,878
Washington DC Segment [Member] | Skyline Properties [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,863	6,984	9,955	13,039
Washington DC Segment [Member] | Skyline Properties [Member] | Impairment Loss [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	(160,700)	0
Washington DC Segment [Member] | Residential Properties [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,834	10,141	21,241	19,974
Washington DC Segment [Member] | Office [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	68,620	75,493	(26,172)	148,917
Washington DC Segment [Member] | 1750 Pennsylvania Avenue [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		2,067		3,990
Washington DC Segment [Member] | 1750 Pennsylvania Avenue [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		66,442		131,888
Other Segment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	41,633	25,257	68,425	72,805
Other Segment [Member] | Real Estate Fund Investment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	7,544	10,496	12,855	21,046
Other Segment [Member] | The Mart (including trade shows) [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	25,965	22,144	48,993	43,185
Other Segment [Member] | 555 California Street [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,117	12,831	23,732	25,232
Other Segment [Member] | India real estate ventures [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	430	375	1,749	2,216
Other Segment [Member] | Other Investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	14,741	9,424	27,063	16,183
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(24,239)	(23,760)	(54,845)	(59,702)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,471	6,561	12,446	15,323
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)		(14,806)
Other Segment [Member] | UE and residual retail properties discontinued operations [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,483	1,540	3,204	23,797
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,879)	(4,061)	(7,486)	(6,042)
Other Segment [Member] | Net gain on sale of residential condominiums [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	0	714	1,860
Other Segment [Member] | Our share of gains on sale of real estate [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		4,513		4,513
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	60,797	55,270	114,392	107,862
Other Segment [Member] | Income before net realized/unrealized gains [Member] | Real Estate Fund Investment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,526	2,671	3,757	4,285
Other Segment [Member] | Net unrealized/realized gains on investments [Member] | Real Estate Fund Investment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,890	4,916	5,451	10,464
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund Investment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 2,128	$ 2,909	$ 3,647	$ 6,297