Investments in Partially Owned Entities (Summary of Income (Loss) ) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 01, 2014
Schedule Of Equity Method Investments
Our share of net (loss) income	$ 165,389	$ (12,630)	$ (59,861)
Real estate impairment losses	161,165	256	26,518
Total revenue	$ 13,600,000	13,423,000	13,620,000
Alexanders Inc
Schedule Of Equity Method Investments
Equity method ownership percentage	32.40%
Equity in net income (loss)	$ 27,470	24,209	21,287
Management leasing and development fees	6,770	6,869	8,722
Our share of net (loss) income	$ 34,240	31,078	30,009
UE
Schedule Of Equity Method Investments
Equity method ownership percentage	5.40%
Equity in net income (loss)	$ 5,003	2,430	0
Management leasing and development fees	836	1,964	0
Our share of net (loss) income	$ 5,839	4,394	0
Toys R Us
Schedule Of Equity Method Investments
Equity method ownership percentage	32.50%
Equity in net income (loss)	$ 0	0	(4,691)
Non-cash impairment losses	0	0	(75,196)
Management leasing and development fees	2,000	2,500	6,331
Our share of net (loss) income	2,000	2,500	(73,556)
Total revenue				$ 12,645,000
Partially Owned Office Buildings
Schedule Of Equity Method Investments
Our share of net (loss) income	(42,100)	(23,556)	93
India real estate ventures
Schedule Of Equity Method Investments
Non-cash impairment losses	13,962	14,806	5,771
Our share of net (loss) income	$ (18,122)	(18,746)	(8,309)
India real estate ventures | Minimum
Schedule Of Equity Method Investments
Equity method ownership percentage	4.10%
India real estate ventures | Maximum
Schedule Of Equity Method Investments
Equity method ownership percentage	36.50%
PREIT
Schedule Of Equity Method Investments
Equity method ownership percentage	8.00%
Our share of net (loss) income	$ (5,213)	(7,450)	0
Other investments
Schedule Of Equity Method Investments
Our share of net (loss) income	$ 10,673	165	(1,867)
Joint Venture | 85 Tenth Avenue
Schedule Of Equity Method Investments
Equity method ownership percentage	49.90%
Income from the repayment of loans and preferred equity	$ 160,843	0	0
Equity in net income (loss)	17,229	(1,015)	(6,231)
Our share of net (loss) income	178,072	(1,015)	(6,231)
Joint Venture | Partially Owned Office Buildings | 666 Fifth Avenue
Schedule Of Equity Method Investments
Our share of net (loss) income	$ (47,000)	(39,600)
Joint Venture | Partially Owned Office Buildings | 650 Madison Avenue
Schedule Of Equity Method Investments
Write off of below market lease liability		$ 12,800
Joint Venture | Partially Owned Office Buildings | West 57th Street
Schedule Of Equity Method Investments
Our share of accelerated depreciation			14,500
Joint Venture | Other investments | Suffolk Downs
Schedule Of Equity Method Investments
Non-cash impairment losses			10,263
Real estate impairment losses			5,959
Asset Impairment Loan Loss Reserve			$ 4,304