Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 3,999,165	$ 858,113
Fifth Avenue and Times Square JV
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	51.50%
Carrying amount of investments in partially owned entities	$ 3,291,231	0
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 464,109	499,005
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 98,543	107,983
PREIT
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	0	59,491
UE
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	0	45,344
Other investments
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	145,282	146,290
Other Liabilities
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ (60,190)	(109,696)
Other Liabilities | 7 West 34th Street
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	53.00%
Carrying amount of investments in partially owned entities	$ (54,004)	(51,579)
Other Liabilities | 85 Tenth Avenue
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	49.90%
Carrying amount of investments in partially owned entities	$ (6,186)	0
Other Liabilities | 330 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 0	$ (58,117)