Segment Information (New York and Washington DC segments) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Information
EBITDA	$ 357,403	$ 202,906
Operating Segments | New York
Segment Information
EBITDA	272,783	260,499
EBITDA Adjusted	272,783	259,057
Operating Segments | New York | Properties sold and other
Segment Information
Gains (Losses) on Sales of Investment Real Estate	0	(1,442)
Operating Segments | New York | Hotel Pennsylvania
Segment Information
EBITDA	(4,398)	(3,472)
Operating Segments | New York | Alexanders Inc
Segment Information
EBITDA	11,562	11,569
Operating Segments | New York | Office
Segment Information
EBITDA	170,077	156,643
Operating Segments | New York | Retail
Segment Information
EBITDA	89,264	89,409
Operating Segments | New York | Residential
Segment Information
EBITDA	6,278	6,350
Operating Segments | Washington DC
Segment Information
EBITDA	70,649	(85,468)
EBITDA adjustment	0	154,755
EBITDA Adjusted	70,649	69,287
Operating Segments | Washington DC | Properties sold and other
Segment Information
Gains (Losses) on Sales of Investment Real Estate	0	(5,945)
Operating Segments | Washington DC | Skyline Properties
Segment Information
Impairment loss	0	160,700
Operating Segments | Washington DC | Office
Segment Information
EBITDA	57,032	(95,875)
Operating Segments | Washington DC | Office | Skyline Properties
Segment Information
EBITDA	0	(155,083)
Operating Segments | Washington DC | Residential
Segment Information
EBITDA	13,617	10,407
Operating Segments | Washington DC | Office Exclude the Skyline Properties
Segment Information
EBITDA	$ 57,032	$ 59,208