Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($)		12 Months Ended
	Dec. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		$ 15,200,000	$ 168,948,000	$ (9,947,000)
Net gains on sale of real estate and other		3,489,000	5,074,000	65,396,000
Income from the repayment of loans and preferred equity	$ 161,000
Suffolk Downs
Schedule of Equity Method Investments [Line Items]
Net gains on sale of real estate and other		15,314,000
Gain on debt investments		11,373,000
Realized gain (loss) on disposal		$ 26,687,000
PREIT
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		8.00%
Non-cash impairment loss		$ (44,465,000)	0	0
Equity in net income (loss)		(8,860,000)	(5,213,000)	(7,450,000)
Our share of net income (loss)		$ (53,325,000)	(5,213,000)	(7,450,000)
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		32.40%
Equity in net income (loss)		$ 25,820,000	27,470,000	24,209,000
Management leasing and development fees		6,033,000	6,770,000	6,869,000
Our share of net income (loss)		$ 31,853,000	$ 34,240,000	31,078,000
UE
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		4.50%	5.40%
Net gain resulting from UE operating partnership unit issuances		$ 21,100,000	$ 0	0
Equity in net income (loss)		5,558,000	5,003,000	2,430,000
Management leasing and development fees		670,000	836,000	1,964,000
Our share of net income (loss)		27,328,000	5,839,000	4,394,000
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		2,020,000	5,773,000	19,808,000
Other investments
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		7,324,000	128,309,000	(57,777,000)
666 Fifth Avenue (Office)
Schedule of Equity Method Investments [Line Items]
Our share of net income (loss)		$ 25,414,000	42,000	37,000
85 Tenth Avenue
Schedule of Equity Method Investments [Line Items]
Debt term	10 years
Mortgage loan interest rate	4.55%
Mortgage loans on real estate, face amount of mortgages	$ 625,000
Net proceeds from loan investments	$ 191,779
India real estate ventures
Schedule of Equity Method Investments [Line Items]
Non-cash impairment loss			$ (13,962,000)	(15,000)
Partially Owned Properties
Schedule of Equity Method Investments [Line Items]
Write off of below market lease liability				$ 12,800,000