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Organization - Additional Information (Detail) | Sep. 30, 2018 property
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Operating Property |
Real Estate Properties [Line Items] |
Number of properties in greater New York City metropolitan area | 7

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- 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

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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
Name: | us-gaap_RealEstatePropertiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis=alx_OperatingPropertyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |