Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 3,491,107	$ 3,999,165
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	$ 2,798,413	3,291,231
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 473,285	464,109
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	32.40%
Carrying amount of investments in partially owned entities	$ 82,902	98,543
Other investments
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ 136,507	145,282
7 West 34th Street
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	53.00%
85 Tenth Avenue
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	49.90%
Other Liabilities
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ (68,420)	(60,190)
Other Liabilities | 7 West 34th Street
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	(55,340)	(54,004)
Other Liabilities | 85 Tenth Avenue
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities	$ (13,080)	$ (6,186)