Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 4,025,534	$ 858,113
Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ (118,338)	(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	$ (60,097)	(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	$ (53,143)	(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	$ (5,098)	0
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	$ 3,306,389	0
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 469,264	499,005
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 103,005	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	$ 146,876	$ 146,290