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Organization - Additional Information (Detail)	Sep. 30, 2017 Property
Operating Property [Member]
Real Estate Properties [Line Items]
Number of properties in greater New York City metropolitan area	7

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

X

- 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

X

- Details

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