Organization and Business - (Narrative) (Details) ft² in Millions	12 Months Ended
Dec. 31, 2017 USD ($) ft² room property residential_unit
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Common limited partnership interest in the Operating Partnership	93.50%
Real estate fund investments
Real Estate Properties
Equity method ownership percentage	25.00%
Alexanders Inc
Real Estate Properties
Equity method ownership percentage	32.40%
Toys R Us
Real Estate Properties
Equity method ownership percentage	32.50%
Total equity method investment \| $	$ 0
New York \| Manhattan \| Office
Real Estate Properties
Square footage of real estate property	20.3
Number of real estate properties \| property	36
New York \| Manhattan \| Retail
Real Estate Properties
Square footage of real estate property	2.7
Number of real estate properties \| property	71
New York \| Manhattan \| Residential Properties
Real Estate Properties
Number of real estate properties \| property	12
Number of units in real estate property (in number of residential units) \| residential_unit	2,009
New York \| Manhattan \| Hotel Pennsylvania
Real Estate Properties
Number of units in real estate property (in number of residential units) \| room	1,700
New York \| Alexanders Inc \| New York City Metropolitan Area
Real Estate Properties
Number of real estate properties \| property	7
Equity method ownership percentage	32.40%
New York \| Alexanders Inc \| New York City Metropolitan Area \| Lexington Avenue 731
Real Estate Properties
Square footage of real estate property	1.3
Other \| Chicago \| the Mart, Chicago
Real Estate Properties
Square footage of real estate property	3.7
Other \| San Francisco \| 555 California Street \| Office
Real Estate Properties
Square footage of real estate property	1.8
Equity method ownership percentage	70.00%
Other \| Toys R Us
Real Estate Properties
Equity method ownership percentage	32.50%
Other \| Toys R Us \| Real estate fund investments
Real Estate Properties
Equity method ownership percentage	25.00%

Area of a real estate property.

No definition available.

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ 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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

+ Details

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- Definition

Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

+ Details

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Period Type:	duration

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

The number of units in a real estate property owned as of the balance sheet date.

+ References

No definition available.

+ Details

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- References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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