Segment Information (Summary of net income and EBITDA reconciliation by segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Information
Total revenues	$ 528,755	$ 502,753	$ 1,547,900	$ 1,489,768
Total expenses	366,520	354,292	1,100,042	1,062,219
Operating income	162,235	148,461	447,858	427,549
(Loss) income from partially owned entities	(41,801)	3,811	5,578	3,892
Loss from real estate fund investments	(6,308)	1,077	(1,649)	28,750
Interest and other investment income, net	9,306	6,459	27,800	20,121
Interest and debt expense	(85,068)	(79,721)	(252,581)	(250,034)
Net gains on disposition of wholly owned and partially owned assets			501	160,225
Income before income taxes	38,364	80,087	227,507	390,503
Income tax expense	(1,188)	(4,563)	(2,429)	(8,921)
Income from continuing operations	37,176	75,524	225,078	381,582
(Loss) income from discontinued operations	(47,930)	25,080	(14,501)	(104,204)
Net (loss) income	(10,754)	100,604	210,577	277,378
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(4,022)	(3,658)	(18,436)	(26,361)
Net (loss) income attributable to the Operating Partnership	(14,776)	96,946	192,141	251,017
Interest and debt expense	113,438	122,979	348,350	376,898
Depreciation and amortization	136,621	172,980	476,406	521,143
Income tax expense (benefit)	1,462	5,102	4,180	13,067
EBITDA	236,745	398,007	1,021,077	1,162,125
General and administrative expenses less $1,975 mark-to-market of our deferred compensation plan	35,495	40,238	131,365	132,085
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(23,304)	(46,500)	(73,125)	(152,023)
Acquisition and transaction related costs	61	1,069	1,073	6,697
Non-cash impairment loss	4,354	1,599	7,572	166,701
Net gain on sale of real estate and other	(1,547)	(5,386)	(21,507)	(168,140)
Net gain resulting from UE operating partnership unit issuances	(5,200)		(21,100)
Our share of net realized/unrealized losses from our real estate fund investments			(18,537)	16,513
Net gain on repayment of Suffolk Downs JV debt investments			(11,373)
NOI	346,241	383,877	1,132,442	1,119,555
Operating Segments | New York
Segment Information
Total revenues	453,609	432,869	1,316,710	1,269,464
Total expenses	284,976	280,689	845,632	818,419
Operating income	168,633	152,180	471,078	451,045
(Loss) income from partially owned entities	1,411	(579)	(954)	(5,143)
Loss from real estate fund investments	0	0	0	0
Interest and other investment income, net	1,413	1,355	4,384	3,684
Interest and debt expense	(61,529)	(51,212)	(179,851)	(162,193)
Net gains on disposition of wholly owned and partially owned assets			0	159,511
Income before income taxes	109,928	101,744	294,657	446,904
Income tax expense	(1,087)	(2,356)	(324)	(4,131)
Income from continuing operations	108,841	99,388	294,333	442,773
(Loss) income from discontinued operations	0	0	0	0
Net (loss) income	108,841	99,388	294,333	442,773
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(2,552)	(2,985)	(8,041)	(9,811)
Net (loss) income attributable to the Operating Partnership	106,289	96,403	286,292	432,962
Interest and debt expense	84,907	66,314	239,032	208,683
Depreciation and amortization	104,799	111,731	328,058	331,448
Income tax expense (benefit)	1,182	2,445	540	4,424
EBITDA	297,177	276,893	853,922	977,517
General and administrative expenses less $1,975 mark-to-market of our deferred compensation plan	9,479	9,783	31,630	27,557
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(21,435)	(35,199)	(58,797)	(114,217)
Non-cash impairment loss	0	0	0	0
Net gain on sale of real estate and other	0	0	0	(159,511)
Net gain resulting from UE operating partnership unit issuances	0		0
Net gain on repayment of Suffolk Downs JV debt investments			0
NOI	280,044	246,588	812,334	716,315
Operating Segments | Other
Segment Information
Total revenues	75,146	69,884	231,190	220,304
Total expenses	81,544	73,603	254,410	243,800
Operating income	(6,398)	(3,719)	(23,220)	(23,496)
(Loss) income from partially owned entities	(43,212)	4,390	6,532	9,035
Loss from real estate fund investments	(6,308)	1,077	(1,649)	28,750
Interest and other investment income, net	7,893	5,104	23,416	16,437
Interest and debt expense	(23,539)	(28,509)	(72,730)	(87,841)
Net gains on disposition of wholly owned and partially owned assets			501	714
Income before income taxes	(71,564)	(21,657)	(67,150)	(56,401)
Income tax expense	(101)	(2,207)	(2,105)	(4,790)
Income from continuing operations	(71,665)	(23,864)	(69,255)	(61,191)
(Loss) income from discontinued operations	(47,930)	25,080	(14,501)	(104,204)
Net (loss) income	(119,595)	1,216	(83,756)	(165,395)
Less net income attributable to noncontrolling interests in consolidated subsidiaries	(1,470)	(673)	(10,395)	(16,550)
Net (loss) income attributable to the Operating Partnership	(121,065)	543	(94,151)	(181,945)
Interest and debt expense	28,531	56,665	109,318	168,215
Depreciation and amortization	31,822	61,249	148,348	189,695
Income tax expense (benefit)	280	2,657	3,640	8,643
EBITDA	(60,432)	121,114	167,155	184,608
General and administrative expenses less $1,975 mark-to-market of our deferred compensation plan	26,016	30,455	99,735	104,528
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(1,869)	(11,301)	(14,328)	(37,806)
Non-cash impairment loss	4,354	1,599	7,572	166,701
Net gain on sale of real estate and other	(1,547)	(5,386)	(21,507)	(8,629)
Net gain resulting from UE operating partnership unit issuances	(5,200)		(21,100)
Net gain on repayment of Suffolk Downs JV debt investments			(11,373)
NOI	66,197	137,289	320,108	403,240
Alexanders Inc
Segment Information
(Loss) income from partially owned entities	7,845	8,785	24,443	25,947
EBITDA	(12,207)	(11,506)	(35,511)	(34,880)
Dividends and distributions received	7,030	6,617	21,090	19,849
Alexanders Inc | Operating Segments | New York
Segment Information
EBITDA	(12,207)	(11,506)	(35,511)	(34,880)
Dividends and distributions received	7,030	6,617	21,090	19,849
Alexanders Inc | Operating Segments | Other
Segment Information
EBITDA	0	0	0	0
Dividends and distributions received	0	0	0	0
Urban Edge Properties
Segment Information
(Loss) income from partially owned entities	6,008	2,158	26,311	4,523
EBITDA	(2,513)	(2,514)	(9,694)	(7,539)
Net gain resulting from UE operating partnership unit issuances	5,200	0	21,100	0
Dividends and distributions received	1,257	1,143	3,773	3,430
Urban Edge Properties | Operating Segments | New York
Segment Information
EBITDA	0	0	0	0
Dividends and distributions received	0	0	0	0
Urban Edge Properties | Operating Segments | Other
Segment Information
EBITDA	(2,513)	(2,514)	(9,694)	(7,539)
Dividends and distributions received	1,257	1,143	3,773	3,430
PREIT
Segment Information
(Loss) income from partially owned entities	(49,748)	52	(53,480)	(4,763)
EBITDA	(3,731)	(3,070)	(15,439)	(8,537)
Non-cash impairment loss	44,465	0	44,465	0
Dividends and distributions received	1,361	1,342	3,929	3,906
PREIT | Operating Segments | New York
Segment Information
EBITDA	0	0	0	0
Non-cash impairment loss	0		0
Dividends and distributions received	0	0	0	0
PREIT | Operating Segments | Other
Segment Information
EBITDA	(3,731)	(3,070)	(15,439)	(8,537)
Non-cash impairment loss	44,465		44,465
Dividends and distributions received	1,361	1,342	3,929	3,906
Including JBGS
Segment Information
Acquisition and transaction related costs	53,642	3,808	68,118	11,319
Including JBGS | Operating Segments | New York
Segment Information
Acquisition and transaction related costs	0	0	0	0
Including JBGS | Operating Segments | Other
Segment Information
Acquisition and transaction related costs	53,642	3,808	68,118	11,319
JBGS
Segment Information
Acquisition and transaction related costs	53,581	2,739	67,045	4,597
Deferred Compensation Plan
Segment Information
General and administrative expenses less $1,975 mark-to-market of our deferred compensation plan	1,975	204	5,233	2,625
Real estate fund investments
Segment Information
Loss from real estate fund investments	(6,308)	1,077	(1,649)	28,750
Our share of net realized/unrealized losses from our real estate fund investments	10,394	99	18,802	(8,741)
Real estate fund investments | Operating Segments | New York
Segment Information
Our share of net realized/unrealized losses from our real estate fund investments	0	0	0	0
Real estate fund investments | Operating Segments | Other
Segment Information
Loss from real estate fund investments	(7,794)	807	(11,333)	13,662
Our share of net realized/unrealized losses from our real estate fund investments	$ 10,394	$ 99	$ 18,802	$ (8,741)