Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($)		3 Months Ended		9 Months Ended
	Mar. 01, 2011	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Equity Method Investments
Our share of net income (loss)		$ 7,206,000	$ (41,801,000)	$ 6,059,000	$ 5,578,000
Gain (loss) on sale of properties				$ 0	3,797,000
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage		32.40%		32.40%
Equity in net income		$ 4,278,000	6,510,000	$ 7,215,000	20,092,000
Management, leasing and development fees		1,149,000	1,335,000	3,378,000	4,351,000
Our share of net income (loss)		$ 5,427,000	7,845,000	$ 10,593,000	24,443,000
UE
Schedule Of Equity Method Investments
Equity method ownership percentage		4.50%		4.50%
Equity in net income		$ 2,696,000	5,908,000	$ 3,017,000	25,793,000
Management, leasing and development fees		67,000	100,000	217,000	518,000
Our share of net income (loss)		2,763,000	6,008,000	3,234,000	26,311,000
Gain from issuance of common operating partnership units			5,200,000		21,100,000
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)		$ 735,000	(967,000)	$ (1,546,000)	79,000
PREIT
Schedule Of Equity Method Investments
Equity method ownership percentage		7.90%		7.90%
Equity in net income			44,465,000
Our share of net income (loss)		$ (616,000)	(49,748,000)	$ (2,113,000)	(53,480,000)
One Park Avenue
Schedule Of Equity Method Investments
Real estate transfer tax	$ 3,000,000			4,978,000
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)		(1,103,000)	(4,939,000)	(4,109,000)	$ 8,225,000
Suffolk Downs
Schedule Of Equity Method Investments
Gain (loss) on sale of properties			15,314,000
Gain on debt investments			11,373,000
Gain recognized on the sale of property and repayment of debt			$ 26,687,000
Alexanders Inc
Schedule Of Equity Method Investments
Chapter 11 bankruptcy relief		1,085,000		1,085,000
Litigation settlement, expense		$ 518,000		518,000
Interest expense				3,162,000
Change in fair value of marketable securities				1,802
Alexanders Inc | Kings Plaza Regional Shopping Center
Schedule Of Equity Method Investments
Real estate transfer tax				$ 7,708,000