Segment Information (New York and Washington DC segments - NOI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Information
NOI	$ 382,349	$ 486,829	$ 722,624	$ 630,379
Operating Segments | New York
Segment Information
NOI adjusted	257,221	229,796	504,586	441,326
NOI adjustments	0	160,284	0	161,743
NOI	257,221	390,080	504,586	603,069
Operating Segments | New York | Alexanders Inc
Segment Information
NOI adjusted	7,029	6,616	14,059	13,233
Operating Segments | New York | Properties sold and other
Segment Information
NOI adjustments	0	773	0	2,232
Operating Segments | New York | Hotel Pennsylvania
Segment Information
NOI adjusted	6,553	3,830	2,187	390
Operating Segments | New York | 7 West 34th Street
Segment Information
Net gain on sale of real estate	0	159,511	0	159,511
Operating Segments | New York | Office
Segment Information
NOI adjusted	158,105	142,639	317,632	277,071
Net operating income allocation		3,931		7,845
Operating Segments | New York | Retail
Segment Information
NOI adjusted	80,193	71,084	159,827	139,433
Net operating income allocation		(3,931)		(7,845)
Operating Segments | New York | Residential
Segment Information
NOI adjusted	5,341	5,627	10,881	11,199
Operating Segments | Washington DC
Segment Information
NOI adjusted	69,623	67,619	135,053	133,363
NOI adjustments	0	3,532	0	(152,837)
NOI	69,623	71,151	135,053	(19,474)
Operating Segments | Washington DC | Properties sold and other
Segment Information
NOI adjustments	0	3,532	0	7,863
Operating Segments | Washington DC | Skyline Properties
Segment Information
Impairment loss	0	0	0	(160,700)
Operating Segments | Washington DC | Office
Segment Information
NOI adjusted	55,592	57,501	107,818	112,937
Operating Segments | Washington DC | Residential
Segment Information
NOI adjusted	$ 14,031	$ 10,118	$ 27,235	$ 20,426