Investments in Partially Owned Entities (Summary of Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 31,426	$ (291,873)	$ 60,499	$ (272,770)
Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Equity method ownership percentage	51.50%		51.50%
Return on preferred equity, net of our share of the expense	$ 9,329	9,330	$ 18,555	18,496
Non-cash impairment loss	0	(306,326)	0	(306,326)
Our share of net income (loss)	19,366	(296,555)	38,198	(281,893)
Equity in net income (loss)	10,037	441	19,643	5,937
Write off of rent receivable		4,737
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 3,758	810	$ 9,730	2,132
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%		32.40%
Management, leasing and development fees	$ 1,962	1,222	$ 2,537	2,482
Our share of net income (loss)	10,287	5,151	16,591	7,827
Equity in net income (loss)	8,325	3,929	14,054	5,345
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	$ (1,985)	$ (1,279)	$ (4,020)	$ (836)