Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 3,504,328	$ 3,999,165
Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ (65,238)	(60,190)
7 West 34th Street
Schedule Of Equity Method Investments
Equity method ownership percentage	53.00%
7 West 34th Street | Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ (54,096)	(54,004)
85 Tenth Avenue
Schedule Of Equity Method Investments
Equity method ownership percentage	49.90%
85 Tenth Avenue | Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ (11,142)	(6,186)
Fifth Avenue and Times Square JV
Schedule Of Equity Method Investments
Equity method ownership percentage	51.50%
Carrying amount of investments in partially owned entities	$ 2,811,374	3,291,231
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 467,498	464,109
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 84,534	98,543
Other investments
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 140,922	$ 145,282