Investments in Partially Owned Entities (Summary of Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 19,103	$ 7,320
Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Equity method ownership percentage	51.50%
Equity in net income (loss)	$ 5,496	0
Return on preferred equity, net of our share of the expense	9,166	0
Our share of net income (loss)	$ 14,662	0
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Equity in net income (loss)	$ 1,416	5,717
Management, leasing and development fees	1,260	1,057
Our share of net income (loss)	2,676	6,774
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)	1,322	106
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)	$ 443	$ 440