Segment Information (New York Segment - NOI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Information
NOI	$ 346,241	$ 383,877	$ 1,132,442	$ 1,119,555
Operating Segments | New York
Segment Information
NOI adjusted	280,044	246,588	812,334	714,083
NOI	280,044	246,588	812,334	716,315
Operating Segments | New York | Alexanders Inc
Segment Information
NOI adjusted	7,030	6,617	21,090	19,849
Operating Segments | New York | Properties sold and other
Segment Information
NOI	0	0	0	2,232
Operating Segments | New York | Hotel Pennsylvania
Segment Information
NOI adjusted	6,252	4,625	9,746	6,390
Operating Segments | New York | Office
Segment Information
NOI adjusted	179,505	157,643	523,531	459,509
Operating Segments | New York | Retail
Segment Information
NOI adjusted	81,839	72,178	241,667	211,611
Operating Segments | New York | Residential
Segment Information
NOI adjusted	$ 5,418	$ 5,525	$ 16,300	$ 16,724