Name:	alx_AsLessorAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	alx_LeaseExtensionPeriod
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	alx_LeasesLeaseTermsAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Approximate rental term range for all outstanding leases to tenants in retail centers and office buildings.

No definition available.

Name:	alx_LeasesOfLessorRentalTermRange
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The remaining number of extension options under a ground lease

No definition available.

Name:	alx_NumberOfExtensionOptions
Namespace Prefix:	alx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

No definition available.

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

This element represents the rental revenue earned during the period from lessee-operators based on revenues generated in their operations, generally in excess of a base amount. Such rental revenue is generally stipulated in the lease agreement, usually will provide for a fixed percentage of revenue to be paid as additional (or possibly only) rent due the lessor, and may be based on gross revenues, net revenues, or multiple variations thereof. Percentage rent is often required under leases with retail outlets located on premises owned by hoteliers, cruise lines, others in the hospitality industry, and shopping mall operators, among others.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Name:	us-gaap_PercentageRent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The aggregate revenue from real estate operations during the reporting period.

Name:	us-gaap_RealEstateRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
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Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_AlexanderApartmentTowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=us-gaap_ApartmentBuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_FlushingPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	alx_ExtensionOptionTermsAxis=alx_TenYearExtensionOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MajorCustomersAxis=alx_BloombergMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: