Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands		12 Months Ended
	Apr. 18, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Non-cash impairment loss		$ 19,098	$ 7,880	$ 236,286
Equity in net income (loss)		(461,351)	130,517	(329,112)
Our share of net income (loss)		(461,351)	130,517	(329,112)
Net gains on disposition of wholly owned and partially owned assets		$ 100,625	50,770	381,320
Fifth Avenue and Times Square JV
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		51.50%
Non-cash impairment loss		$ (489,859)	0	(413,349)
Equity in net income (loss)		55,248	47,144	21,063
Return on preferred equity, net of our share of the expense		37,416	37,416	37,357
Our share of net income (loss)		$ (397,195)	84,560	(354,929)
Net gains on disposition of wholly owned and partially owned assets	$ 13,613
Alexander's
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		32.40%
Equity in net income (loss)		$ 18,439	20,116	13,326
Net gain on sale of land		0	14,576	0
Management, leasing and development fees		4,534	5,429	5,309
Our share of net income (loss)		22,973	40,121	18,635
Write off of rent receivable				4,846
Partially Owned Office Buildings/Land
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		(110,261)	6,384	11,943
Other investments
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		23,132	$ (548)	$ (4,761)
Net gains on disposition of wholly owned and partially owned assets		17,185
650 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Non-cash impairment loss		$ 93,353