Segment Information (New York and Washington DC segments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Information
EBITDA	$ 2,173,985	$ 1,852,766	$ 2,277,084
Operating Segments | New York
Segment Information
EBITDA	1,268,696	1,250,819	1,439,189
Operating Segments | New York | Adjustment
Segment Information
EBITDA	(162,631)	(179,978)	(480,451)
Operating Segments | New York | Adjusted
Segment Information
EBITDA	1,106,065	1,070,841	958,738
Operating Segments | New York | Net Gain On Sale Of Real Estate | Adjustment
Segment Information
EBITDA	(159,511)	(142,693)	(440,537)
Operating Segments | New York | Discontinued operations and sold properties | Adjustment
Segment Information
EBITDA	(3,120)	(35,985)	(39,743)
Operating Segments | New York | Other | Adjustment
Segment Information
EBITDA	0	(1,300)	(171)
Operating Segments | New York | Hotel Pennsylvania
Segment Information
EBITDA	10,007	23,044	30,753
Operating Segments | New York | Alexanders Inc
Segment Information
EBITDA	46,182	42,858	41,746
Operating Segments | New York | Office
Segment Information
EBITDA	805,708	804,272	1,063,355
Operating Segments | New York | Retail
Segment Information
EBITDA	381,739	358,379	281,428
Operating Segments | New York | Residential
Segment Information
EBITDA	25,060	22,266	21,907
Operating Segments | Washington DC
Segment Information
EBITDA	655,109	426,175	331,101
Operating Segments | Washington DC | Adjustment
Segment Information
EBITDA	(364,610)	(135,604)	(40,676)
Operating Segments | Washington DC | Adjusted
Segment Information
EBITDA	290,499	290,571	290,425
Operating Segments | Washington DC | Net Gain On Sale Of Real Estate | Adjustment
Segment Information
EBITDA	(15,302)	(102,404)	(1,800)
Operating Segments | Washington DC | Discontinued operations and sold properties | Adjustment
Segment Information
EBITDA	(22,131)	(33,605)	(38,876)
Operating Segments | Washington DC | Other | Adjustment
Segment Information
EBITDA	0	405	0
Operating Segments | Washington DC | Skyline Properties | Impairment Loss | Adjustment
Segment Information
EBITDA	160,700	0	0
Operating Segments | Washington DC | Skyline Properties | Net gain on extinguishment of debt | Adjustment
Segment Information
EBITDA	(487,877)	0	0
Operating Segments | Washington DC | Office
Segment Information
EBITDA	608,452	385,388	289,520
Operating Segments | Washington DC | Office | Skyline Properties
Segment Information
EBITDA	348,016	26,325	29,250
Operating Segments | Washington DC | Residential
Segment Information
EBITDA	46,657	40,787	41,581
Operating Segments | Washington DC | Office Exclude the Skyline Properties
Segment Information
EBITDA	$ 260,436	$ 359,063	$ 260,270