Segment Information (Parentheticals) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Sep. 30, 2014
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 374,463,000	$ 469,839,000
Carrying amount of investments in partially owned entitiies	1,408,214,000		1,246,496,000
Impairment losses	256,000	20,842,000
Mark-to-market of investments in our deferred compensation plan	2,859,000	4,400,000
Transaction Costs Spin Off	22,645,000	499,000
Out Performance Plan [Member]
Segment Reporting Information [Line Items]
Additional Expense From Acceleration Of Vesting	8,817,000
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
Impairment losses		75,196,000
Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Mark-to-market of investments in our deferred compensation plan	2,859,000	4,400,000
Corporate General And Administrative Expenses For Remainder Of 2015	2,600,000
Corporate General And Administrative Expenses After 2015	6,217,000
Retail Segment Strip Shopping Centers [Member]
Segment Reporting Information [Line Items]
Transaction Costs Spin Off	22,645,000	499,000
New York Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	248,945,000	233,798,000
New York Segment [Member] | Alexanders Inc [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,407,000	10,430,000
New York Segment [Member] | Hotel Pennsylvania [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(2,126,000)	(706,000)
New York Segment [Member] | Retail Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	81,305,000	66,195,000
New York Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	159,359,000	157,879,000
Washington DC Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	83,257,000	84,087,000
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	67,385,000	67,257,000
Washington DC Segment [Member] | Washington DC Skyline Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,055,000	6,499,000
Washington DC Segment [Member] | Washington Dc Residential Property [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,817,000	10,331,000
Washington DC Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	73,440,000	73,756,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	42,261,000	151,954,000
Other Segment [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,550,000	7,299,000
Other Segment [Member] | Merchandise Mart Other [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	21,041,000	19,087,000
Other Segment [Member] | Other Properties 555 California Street [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,401,000	12,066,000
Other Segment [Member] | India real estate ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,841,000	1,824,000
Other Segment [Member] | Our share of Toys "R" Us [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	83,550,000
Other Segment [Member] | Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	9,109,000	9,447,000
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(35,942,000)	(25,982,000)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,762,000	8,073,000
Other Segment [Member] | Urban Edge Properties And Residual Retail Properties Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	19,907,000	32,100,000
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(1,981,000)	(1,285,000)
Other Segment [Member] | Net Gain On Sale Of Marketable Securities Land Parcels And Residential Condominimums [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,860,000	9,635,000
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	(5,287,000)	(3,860,000)
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	54,942,000	133,273,000
Other Segment [Member] | Income before net realized / unrealized gains (Operations) [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	1,614,000	1,982,000
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	3,388,000	1,775,000
Other Segment [Member] | Unrealized Gain From Mark To Market [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 5,548,000	$ 3,542,000