Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 730,264	$ 858,113
Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ (117,375)	(109,696)
330 Madison Avenue | Other liabilities
Schedule Of Equity Method Investments
Equity method ownership percentage	25.00%
Carrying amount of investments in partially owned entities	$ (60,054)	(58,117)
7 West 34th Street | Other liabilities
Schedule Of Equity Method Investments
Equity method ownership percentage	53.00%
Carrying amount of investments in partially owned entities	$ (51,464)	(51,579)
85 Tenth Avenue | Other liabilities
Schedule Of Equity Method Investments
Equity method ownership percentage	49.90%
Carrying amount of investments in partially owned entities	$ (5,857)	0
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 478,240	499,005
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 106,786	107,983
PREIT
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	0	59,491
UE
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	0	45,344
Other investments
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 145,238	$ 146,290