Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)	$ 78,865	$ 9,149	$ 15,200
Transfer tax expense	$ 0	13,103	0
Suffolk Downs
Schedule of Equity Method Investments [Line Items]
Net gains on sale of real estate and other			15,314
Gain on debt investments			11,373
Realized gain (loss) on disposal			26,687
Fifth Avenue and Times Square JV
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	51.50%
Equity in net income (loss)	$ 31,130	0	0
Return on preferred equity, net of our share of the expense	27,586	0	0
Our share of net income (loss)	$ 58,716	0	0
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	32.40%
Equity in net income (loss)	$ 19,204	10,485	25,820
Management, leasing and development fees	4,575	4,560	6,033
Our share of net income (loss)	23,779	15,045	31,853
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)	(3,443)	(3,085)	2,109
Other investments
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)	$ (187)	(2,811)	(18,762)
One Park Avenue
Schedule of Equity Method Investments [Line Items]
Transfer tax expense		4,978
666 Fifth Avenue (Office)
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)		(4,873)	(25,414)
PREIT
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)			(44,465)
Urban Edge Properties
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)			$ 21,100
Alexanders Inc | Kings Plaza Regional Shopping Center
Schedule of Equity Method Investments [Line Items]
Transfer tax expense		$ 7,708