Investments in Partially Owned Entities (Summary of Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule Of Equity Method Investments
Non-cash impairment loss			$ 0	$ (26,140)
Our share of net income (loss)	$ (291,873)	$ 22,873	(272,770)	30,193
Write off of rent receivable	$ 4,737		$ 4,737
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)	0	$ (306,326)	0
Return on preferred equity, net of our share of the expense	9,330	8,586	18,496	8,586
Equity in net income (loss)	441	11,217	5,937	11,217
Our share of net income (loss)	$ (296,555)	19,803	$ (281,893)	19,803
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%		32.40%
Equity in net income (loss)	$ 3,929	3,597	$ 5,345	9,314
Management, leasing and development fees	1,222	1,122	2,482	2,179
Our share of net income (loss)	5,151	4,719	7,827	11,493
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	810	(1,451)	2,132	(1,345)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	(1,279)	$ (198)	(836)	$ 242
1540 Broadway
Schedule Of Equity Method Investments
Rent reduction	$ 4,360		$ 4,360