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Organization (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2012 segment property
Operating Properties [Abstract] |
Number of properties in greater New York City metropolitan area | 6
Number Of Operating Segments | 1
731 Lexington Avenue [Member] |
Operating Properties [Abstract] |
Area of property | 1,307,000
731 Lexington Avenue [Member] | Office Space [Member] |
Operating Properties [Abstract] |
Area of property | 885,000
731 Lexington Avenue [Member] | Office Space [Member] | Tenant Occupant [Member] | Bloomberg [Member] |
Operating Properties [Abstract] |
Area of property | 885,000
731 Lexington Avenue [Member] | Retail Space [Member] |
Operating Properties [Abstract] |
Area of property | 174,000
731 Lexington Avenue [Member] | Retail Space [Member] | Tenant Occupant [Member] | Container Store [Member] |
Operating Properties [Abstract] |
Area of property | 34,000
731 Lexington Avenue [Member] | Retail Space [Member] | Tenant Occupant [Member] | Hennes Mauritz [Member] |
Operating Properties [Abstract] |
Area of property | 27,000
731 Lexington Avenue [Member] | Retail Space [Member] | Tenant Occupant [Member] | Home Depot [Member] |
Operating Properties [Abstract] |
Area of property | 83,000
731 Lexington Avenue [Member] | Residential Space [Member] |
Operating Properties [Abstract] |
Number Of Residential Units Sold | 105
Area of property | 248,000
Rego Park I [Member] |
Operating Properties [Abstract] |
Area of property | 343,000
Rego Park I [Member] | Tenant Occupant [Member] | Burlington Coat Factory [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 50,000
Rego Park I [Member] | Tenant Occupant [Member] | Marshalls [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 36,000
Rego Park I [Member] | Tenant Occupant [Member] | Bed Bath Beyond [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 46,000
Rego Park I [Member] | Tenant Occupant [Member] | Sears [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 195,000
Rego Park II [Member] |
Operating Properties [Abstract] |
Area of property | 610,000
Rego Park II [Member] | Tenant Occupant [Member] | Toys R Us [Member] |
Operating Properties [Abstract] |
Related Party Ownership In Tenant | one-third
Rego Park II [Member] | Tenant Occupant [Member] | Toys R Us [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 47,000
Rego Park II [Member] | Tenant Occupant [Member] | Kohls [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 133,000
Rego Park II [Member] | Tenant Occupant [Member] | Century 21 [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 135,000
Rego Park II [Member] | Tenant Occupant [Member] | Costco [Member] | Anchor [Member] |
Operating Properties [Abstract] |
Area of property | 145,000
Rego Park II [Member] | Apartment Tower [Member] | Properties to be developed [Member] |
Operating Properties [Abstract] |
Area of property | 250,000
Approximate Number Of Apartment Units | 300
Rego Park II [Member] | Apartment Tower [Member] | Maximum [Member] | Properties to be developed [Member] |
Operating Properties [Abstract] |
Estimated future funding | 120,000,000
Rego Park II [Member] | Apartment Tower [Member] | Minimum [Member] | Properties to be developed [Member] |
Operating Properties [Abstract] |
Estimated future funding | 100,000,000
Paramus [Member] | Tenant Occupant [Member] | Ikea [Member] |
Operating Properties [Abstract] |
Area Of Land | 30.3
Flushing [Member] | Tenant Occupant [Member] | New World Mall Llc [Member] |
Operating Properties [Abstract] |
Area of property | 167,000
Rego Park III [Member] | Property To Be Developed [Member] |
Operating Properties [Abstract] |
Area Of Land | 3.4

X
- Definition The approximate unit of apartment units + References No definition available. + Details
Name: | alx_ApproximateNumberOfApartmentUnits
Namespace Prefix: | alx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated future funding for proposed development + References No definition available. + Details
Name: | alx_EstimatedFutureFunding
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of residential units sold on the property + References No definition available. + Details
Name: | alx_NumberOfResidentialUnitsSold
Namespace Prefix: | alx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | alx_OperatingPropertiesAbstract
Namespace Prefix: | alx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Vornado's Ownership interest in tenants occupying space at the company's properties + References No definition available. + Details
Name: | alx_RelatedPartyOwnershipInTenant
Namespace Prefix: | alx_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Area of land held. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | instant

X
- Definition Area of a real estate property. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of real estate properties owned as of the balance sheet date. + References No definition available. + Details
Name: | us-gaap_NumberOfRealEstateProperties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant