Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 4,023,820	$ 858,113
Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	(58,036)	(109,696)
330 Madison Avenue | Other liabilities
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 0	(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	$ (52,222)	(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	$ (5,814)	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	$ 3,308,363	0
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Carrying amount of investments in partially owned entities	$ 467,787	499,005
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 101,228	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	$ 146,442	$ 146,290