Investments in Partially Owned Entities (Alexander's Inc.) (Details) - Alexanders Inc - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
Nov. 30, 2012	Mar. 31, 2018
Equity Method Investments And Income From Equity Method Investments
Ownership common shares, investee (in shares)		1,654,068
Equity method ownership percentage		32.40%
Closing share price (in dollars per share)		$ 381.23
Equity method investment fair value		$ 630,580
Excess of investee's fair value over carrying amount		514,502
Excess of investee's carrying amount over equity in net assets		39,317
Additional real estate transfer tax		7,708
Kings Plaza Regional Shopping Center
Equity Method Investments And Income From Equity Method Investments
Real estate transfer tax	$ 3,971
Additional real estate transfer tax		$ 23,797
Vornado Realty Trust
Equity Method Investments And Income From Equity Method Investments
Equity method ownership percentage		32.40%

Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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This item represents the aggregate value of each identified investment accounted for under the equity method of accounting based on the quoted market price for those investments in common stock for which a quoted market price is available.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
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Balance held at close of period in number of shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-12C.Column B)
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A tax based on the assessed value of real estate by the local government. The tax is usually based on the value of property (including the land).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 720
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-Paragraph 3
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Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

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Additional Real Estate Tax Expense

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Equity Method Investment Fair Value In Excess Of Carrying Amount

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This abstract relates to equity method investments and income from equity method investments

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