Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 46,276	$ 642	$ 47,721	$ (3,598)
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%		32.40%
Equity in net income (loss)	$ 6,690	6,812	$ 13,582	13,749
Management leasing and development fees	1,507	1,688	3,016	3,413
Our share of net income (loss)	$ 8,197	8,500	$ 16,598	17,162
UE
Schedule Of Equity Method Investments
Equity method ownership percentage	4.80%		4.80%		5.40%
Gain (loss) from issuance of common operating partnership units	$ 15,900	0	$ 15,900	0
Equity in net income (loss)	2,894	1,071	3,985	1,947
Management leasing and development fees	209	209	418	418
Our share of net income (loss)	19,003	1,280	20,303	2,365
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	(7,897)	(12,398)	(17,840)	(26,647)
India real estate ventures
Schedule Of Equity Method Investments
Our share of net income (loss)	$ (1,644)	(1,934)	$ 10	(2,620)
India real estate ventures | Minimum
Schedule Of Equity Method Investments
Equity method ownership percentage	4.10%		4.10%
India real estate ventures | Maximum
Schedule Of Equity Method Investments
Equity method ownership percentage	36.50%		36.50%
PREIT
Schedule Of Equity Method Investments
Equity method ownership percentage	8.00%		8.00%
Our share of net income (loss)	$ (902)	(527)	$ (3,732)	(4,815)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 29,519	$ 5,721	$ 32,382	$ 10,957