Segment Information (Parentheticals) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 210,393	$ 374,463
Mark-to-market (loss) income of investments in our deferred compensation plan	(1,938)	2,859
Additional Expense From Acceleration Of Vesting		4,542
Transaction Costs Spin Off	0	22,645
Corporate General And Administrative Expenses [Member]
Segment Information [Line Items]
Mark-to-market (loss) income of investments in our deferred compensation plan	(1,938)	2,859
Retail Segment Strip Shopping Centers [Member]
Segment Information [Line Items]
Transaction Costs Spin Off		22,645
New York Segment [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	260,499	248,945
New York Segment [Member] | Alexanders Inc [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,569	10,407
New York Segment [Member] | Hotel Pennsylvania [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(3,472)	(2,126)
New York Segment [Member] | Residential Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,350	5,050
New York Segment [Member] | Retail Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	93,323	81,305
New York Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	152,729	154,309
New York Segment [Member] | 20 Broad Street [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		3,540
New York Segment [Member] | New York Segment Excluding Broad Street [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		150,769
Washington DC Segment [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(84,385)	83,257
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	61,988	67,385
Washington DC Segment [Member] | Skyline Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,092	6,055
Washington DC Segment [Member] | Skyline Properties [Member] | Impairment Loss [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(160,700)	0
Washington DC Segment [Member] | Residential Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,235	9,817
Washington DC Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(93,620)	73,440
Washington DC Segment [Member] | 1750 Pennsylvania Avenue [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		1,923
Washington DC Segment [Member] | Washington DC Office Excluding Skyline And Pennsylvania Properties [Member] | Operating Segments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization		65,462
Other Segment [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	34,279	42,261
Other Segment [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	5,311	10,550
Other Segment [Member] | The Mart and trade shows [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	23,028	21,041
Other Segment [Member] | 555 California Street [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,615	12,401
Other Segment [Member] | India real estate ventures [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,319	1,841
Other Segment [Member] | Other Investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,322	6,759
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(30,606)	(35,942)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,975	8,762
Other Segment [Member] | UE and residual retail properties discontinued operations [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	721	22,257
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(4,607)	(1,981)
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	714	1,860
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	7,487	(5,287)
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	53,595	52,592
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	2,231	1,614
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	1,561	5,548
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 1,519	$ 3,388