Segment Information (New York and Washington DC segments - EBITDA) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Information
EBITDA	$ 426,929	$ 561,212	$ 784,332	$ 764,118
Operating Segments | New York
Segment Information
EBITDA Adjusted	283,962	278,936	556,745	537,993
EBITDA adjustment	0	161,189	0	162,631
EBITDA	283,962	440,125	556,745	700,624
Operating Segments | New York | Properties sold and other
Segment Information
EBITDA adjustment	0	1,678	0	3,120
Operating Segments | New York | Hotel Pennsylvania
Segment Information
EBITDA Adjusted	6,520	3,797	2,122	325
Operating Segments | New York | 7 West 34th Street
Segment Information
Net gain on sale of real estate	$ 0	159,511	$ 0	159,511
Equity method ownership percentage	47.00%		47.00%
Operating Segments | New York | Alexanders Inc
Segment Information
EBITDA Adjusted	$ 11,742	11,805	$ 23,304	23,374
Operating Segments | New York | Office
Segment Information
EBITDA Adjusted	169,327	165,576	339,405	320,585
Operating Segments | New York | Retail
Segment Information
EBITDA Adjusted	90,183	91,421	179,446	181,022
Operating Segments | New York | Residential
Segment Information
EBITDA Adjusted	6,190	6,337	12,468	12,687
Operating Segments | Washington DC
Segment Information
EBITDA Adjusted	71,449	71,475	140,945	139,802
EBITDA adjustment	0	5,743	0	(149,012)
EBITDA	71,449	77,218	140,945	(9,210)
Operating Segments | Washington DC | Properties sold and other
Segment Information
EBITDA adjustment	0	5,743	0	11,688
Operating Segments | Washington DC | Skyline Properties
Segment Information
Impairment loss	0	0	0	(160,700)
Operating Segments | Washington DC | Office
Segment Information
EBITDA Adjusted	57,418	61,357	113,710	119,376
Operating Segments | Washington DC | Residential
Segment Information
EBITDA Adjusted	$ 14,031	$ 10,118	$ 27,235	$ 20,426