Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 25,946	$ 7,206	$ 56,139	$ 6,059
Transfer tax	$ 0	0	$ 0	13,103
Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Equity method ownership percentage	51.50%		51.50%
Equity in net income (loss)	$ 9,891	0	$ 21,108	0
Return on preferred equity, net of our share of the expense	9,545	0	18,131	0
Our share of net income (loss)	$ 19,436	0	$ 39,239	0
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%		32.40%
Equity in net income (loss)	$ 5,393	4,278	$ 14,707	7,215
Management, leasing and development fees	1,299	1,149	3,478	3,378
Our share of net income (loss)	6,692	5,427	18,185	10,593
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	(186)	735	(1,531)	(1,546)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 4	$ 1,044	246	(2,988)
280 Park Avenue
Schedule Of Equity Method Investments
Our share of net income (loss)			$ (1,079)
One Park Avenue
Schedule Of Equity Method Investments
Transfer tax				4,978
Alexanders Inc | Kings Plaza Regional Shopping Center
Schedule Of Equity Method Investments
Transfer tax				$ 7,708