Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
	Mar. 01, 2011	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)			$ 9,149	$ 15,200	$ 168,948
Net gains on sale of real estate and other			0	3,489	5,074
Long-term Debt			$ 1,668,823	1,592,348
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
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage			32.40%
Equity in net income (loss)			$ 10,485	25,820	27,470
Management leasing and development fees			4,560	6,033	6,770
Our share of net income (loss)			$ 15,045	31,853	34,240
UE
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage			4.50%
Equity in net income (loss)			$ 4,227	26,658	5,003
Management leasing and development fees			233	670	836
Our share of net income (loss)			4,460	27,328	5,839
Net gain resulting from UE operating partnership unit issuances				21,100
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)			$ (3,085)	2,109	5,773
PREIT
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage			7.90%
Our share of net income (loss)			$ (3,015)	(53,325)	(5,213)
Non-cash impairment loss				44,465
Other investments
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)			(4,256)	7,235	128,309
One Park Avenue
Schedule of Equity Method Investments [Line Items]
Real estate tax expense	$ 3,000		4,978
666 Fifth Avenue (Office)
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)			4,873	$ 25,414	41,532
85 Tenth Avenue
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss)					$ 160,843
Debt term					10 years
Debt instrument, interest rate, stated percentage					4.55%
Long-term Debt					$ 625,000
Debt repayment					$ 191,779
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Non-cash straight line rent write off			1,085
Litigation settlement, expense			518
Interest expense		$ 3,882
Alexanders Inc | Kings Plaza Regional Shopping Center
Schedule of Equity Method Investments [Line Items]
Real estate tax expense			$ 7,708