Segment Information (Parentheticals) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Sep. 30, 2014
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	$ 376,681	$ 449,397	$ 751,144	$ 919,236
Carrying amount of investments in partially owned entitiies	1,477,090		1,477,090		$ 1,246,496
Impairment losses			256	20,842
Mark-to-market of investments in our deferred compensation plan	(609)	2,380	2,250	6,780
Transaction Costs Spin Off	327	3,016	22,972	3,515
Out Performance Plan [Member]
Segment Reporting Information [Line Items]
Additional Expense From Acceleration Of Vesting			$ 8,817
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
Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Mark-to-market of investments in our deferred compensation plan	609	2,380	$ 2,250	6,780
Corporate General And Administrative Expenses For Remainder Of 2015			1,734
Corporate General And Administrative Expenses After 2015			6,217
Retail Segment Strip Shopping Centers [Member]
Segment Reporting Information [Line Items]
Transaction Costs Spin Off	327	3,016	22,972	3,515
New York Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	275,988	251,329	524,933	485,127
New York Segment [Member] | Alexanders Inc [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,241	10,271	20,648	20,701
New York Segment [Member] | Hotel Pennsylvania [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	8,856	10,278	6,730	9,572
New York Segment [Member] | Retail Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	86,151	67,947	167,456	134,142
New York Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	170,740	162,833	330,099	320,712
Washington DC Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	85,634	84,894	168,891	168,981
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	68,514	67,057	135,898	134,314
Washington DC Segment [Member] | Washington DC Skyline Properties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,984	7,073	13,039	13,572
Washington DC Segment [Member] | Washington Dc Residential Property [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,136	10,764	19,954	21,095
Washington DC Segment [Member] | Office [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	75,498	74,130	148,937	147,886
Other Segment [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	15,059	113,174	57,320	265,128
Other Segment [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	10,496	38,330	21,046	45,629
Other Segment [Member] | Merchandise Mart Other [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	22,144	22,454	43,185	41,541
Other Segment [Member] | Other Properties 555 California Street [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	12,831	11,506	25,232	23,572
Other Segment [Member] | India real estate ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	375	99	2,216	1,923
Other Segment [Member] | Our share of Toys "R" Us [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	500	5,189	1,954	90,586
Other Segment [Member] | Other Investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	11,222	6,780	18,966	14,380
Other Segment [Member] | Corporate General And Administrative Expenses [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(23,760)	(23,022)	(59,702)	(49,004)
Other Segment [Member] | Investment Income and other, net [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	6,561	8,032	15,323	16,105
Other Segment [Member] | Our Share Of Impairment Loss On India Real Estate Ventures [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(14,806)	0	(14,806)	0
Other Segment [Member] | Our share of gains on sale of real estate of partially owned entities [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	4,513	0	4,513	0
Other Segment [Member] | Acquisition and transaction related costs [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(4,061)	(1,067)	(6,042)	(2,352)
Other Segment [Member] | UE and residual retail properties discontinued operations [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	(758)	48,672	19,060	80,772
Other Segment [Member] | Net gain on sale of residential condominiums and a land parcel
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	0	905	1,860	10,540
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	(10,198)	(4,704)	(15,485)	(8,564)
Other Segment [Member] | Other EBITDA attributable to identifiable investments [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	57,568	84,358	112,599	217,631
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	1,533	2,191	4,285	4,617
Other Segment [Member] | Carried Interest [Member] | Real Estate Fund [Member]
Segment Reporting Information [Line Items]
Earnings Before Interest, Taxes, Depreciation, and Amortization	2,909	11,874	6,297	13,205
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	$ 6,054	$ 24,265	$ 10,464	$ 27,807