Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Real estate impairment losses	$ (236,286)	$ (8,065)	$ (12,000)
Our share of net income (loss)	(329,112)	78,865	9,149
Transfer tax expense	$ 0	0	13,103
One Park Avenue
Schedule of Equity Method Investments [Line Items]
Transfer tax expense			4,978
Fifth Avenue and Times Square JV
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	51.50%
Real estate impairment losses	$ (413,349)	0	0
Return on preferred equity, net of our share of the expense	37,357	27,586	0
Equity in net income (loss)	21,063	31,130	0
Our share of net income (loss)	(354,929)	58,716	0
Fifth Avenue and Times Square JV | 1540 Broadway
Schedule of Equity Method Investments [Line Items]
Rent reduction	13,971
Write off of rent receivable	$ 3,125
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	32.40%
Equity in net income (loss)	$ 13,326	19,204	10,485
Management, leasing and development fees	5,309	4,575	4,560
Our share of net income (loss)	18,635	23,779	15,045
Write off of rent receivable	4,846
Transfer tax paid			7,708
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)	12,742	(3,443)	(3,085)
Other investments
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)	$ (5,560)	$ (187)	(2,811)
666 Fifth Avenue (Office)
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)			$ (4,873)