Investments in Partially Owned Entities (Summary of Investments) (Details) - USD ($) $ in Thousands	May 27, 2016	Dec. 31, 2018	Oct. 30, 2018	Oct. 29, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities		$ 858,113			$ 1,056,829
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities		$ 499,005			504,393
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		32.40%
Carrying amount of investments in partially owned entities		$ 107,983			126,400
PREIT
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		7.90%
Carrying amount of investments in partially owned entities		$ 59,491			66,572
UE
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		4.50%
Carrying amount of investments in partially owned entities		$ 45,344			46,152
Other investments
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities		$ 146,290			313,312
Farley Office and Retail Building
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage			95.00%	50.10%
Equity method additional ownership percentage		44.90%	44.90%
Other Liabilities
Schedule of Equity Method Investments [Line Items]
Carrying amount of investments in partially owned entities		$ (109,696)			(101,368)
Other Liabilities | Three Three Zero Madison Avenue
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		25.00%
Carrying amount of investments in partially owned entities		$ (58,117)			(53,999)
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		$ (51,579)			$ (47,369)
Sale of ownership interest percent	47.00%