Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($)		3 Months Ended		6 Months Ended
	Mar. 01, 2011	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Equity Method Investments
Our share of net income (loss)		$ 8,757,000	$ 46,021,000	$ (1,147,000)	$ 47,379,000
Gain (loss) on sale of properties				$ 0	2,267,000
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage		32.40%		32.40%
Equity in net income		$ 6,146,000	6,690,000	$ 2,937,000	13,582,000
Our share of net income (loss)		7,167,000	8,197,000	5,166,000	16,598,000
Management, leasing and development fees		1,021,000	1,507,000	2,229,000	3,016,000
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net income (loss)		$ 2,002,000	236,000	$ (2,281,000)	1,046,000
UE
Schedule Of Equity Method Investments
Equity method ownership percentage		4.50%		4.50%
Equity in net income		$ 1,038,000	18,794,000	$ 321,000	19,885,000
Our share of net income (loss)		1,112,000	19,003,000	471,000	20,303,000
Management, leasing and development fees		$ 74,000	209,000	$ 150,000	418,000
Gain from issuance of common operating partnership units					15,900,000
PREIT
Schedule Of Equity Method Investments
Equity method ownership percentage		8.00%		8.00%
Equity in net income		$ (1,068,000)	(902,000)	$ (1,497,000)	(3,732,000)
Other investments
Schedule Of Equity Method Investments
Our share of net income (loss)		$ (456,000)	19,487,000	(3,006,000)	$ 13,164,000
One Park Avenue
Schedule Of Equity Method Investments
Real estate transfer tax	$ 3,000,000			4,978,000
Kings Plaza Regional Shopping Center
Schedule Of Equity Method Investments
Real estate transfer tax				$ 7,708,000
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