Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule Of Equity Method Investments
Non-cash impairment loss	$ (4,354)	$ (1,599)	$ (7,572)	$ (166,701)
Our share of net income (loss)	(41,801)	3,811	5,578	3,892
Net gain resulting from UE operating partnership unit issuances	$ (5,200)		$ (21,100)
PREIT
Schedule Of Equity Method Investments
Equity method ownership percentage	8.00%		8.00%
Non-cash impairment loss	$ (44,465)	0	$ (44,465)	0
Equity in earnings	(5,283)	52	(9,015)	(4,763)
Our share of net income (loss)	$ (49,748)	52	$ (53,480)	(4,763)
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%		32.40%
Equity in earnings	$ 6,510	6,891	$ 20,092	20,640
Our share of net income (loss)	7,845	8,785	24,443	25,947
Management, leasing and development fees	$ 1,335	1,894	$ 4,351	5,307
UE
Schedule Of Equity Method Investments
Equity method ownership percentage	4.50%		4.50%		5.40%
Equity in earnings	$ 708	1,949	$ 4,693	3,896
Our share of net income (loss)	6,008	2,158	26,311	4,523
Management, leasing and development fees	100	209	518	627
Net gain resulting from UE operating partnership unit issuances	5,200	0	21,100	0
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	(5,551)	(8,642)	(23,508)	(29,882)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	$ (355)	$ 1,458	$ 31,812	$ 8,067