Investments in Partially Owned Entities (Summary of Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule Of Equity Method Investments
Non-cash impairment loss					$ 0	$ (26,140)
Our share of net income (loss)		$ (80,909)		$ 25,946	$ (353,679)	56,139
Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Equity method ownership percentage		51.50%			51.50%
Non-cash impairment loss	$ (306,326)	$ (107,023)	$ (306,326)	0	$ (413,349)	0
Return on preferred equity, net of our share of the expense		9,430		9,545	27,926	18,131
Equity in net income (loss)		7,694		9,891	13,631	21,108
Our share of net income (loss)		$ (89,899)		19,436	$ (371,792)	39,239
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage		32.40%			32.40%
Equity in net income (loss)		$ 2,075		5,393	$ 7,420	14,707
Management, leasing and development fees		1,296		1,299	3,778	3,478
Our share of net income (loss)		3,371		6,692	11,198	18,185
Write off of rent receivable		3,139			4,846
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)		6,418		(186)	8,550	(1,531)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)		$ (799)		$ 4	(1,635)	$ 246
1540 Broadway | Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Write off of rent receivable					2,997
Rent reduction					$ 10,047