Proceeds From Sale Of Land Held-For-Investment, Excluding Disposal Cost

No definition available.

Name:	alx_ProceedsFromSaleOfLandHeldForInvestmentExcludingDisposalCost
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Proceeds from Sale of Land Held-for-investment, Net

+ References

No definition available.

+ Details

Name:	alx_ProceedsFromSaleOfLandHeldForInvestmentNet
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Area of land held.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfLand
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

The net gain (loss) resulting from sales and other disposals of real estate owned for investment purposes.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(a)(8))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(7)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(9)(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(14)(d))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-SubTopic 360
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479638/944-360-45-3

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 4
-SubTopic 360
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479638/944-360-45-4

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 360
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479607/944-360-50-1

+ Details

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

- Definition

The cash inflow from the sale of land held for investment; excludes sales of land held as inventory or productive use.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-12

+ Details

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

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 976
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482856/976-310-50-1

+ Details

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

- Definition

Amount of cash outflow for short-term and long-term debt. Excludes payment of lease obligation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 15
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-15

+ Details

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

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoParkIIILandParcelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_BronxLandParcelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	alx_RealEstatePropertyOccupantAxis=alx_IkeaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=alx_TenantOccupantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_ParamusPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Real Estate Sales (Details) $ in Thousands				12 Months Ended
Real Estate Sales (Details) $ in Thousands	May 19, 2023 USD ($)	Oct. 04, 2021 USD ($) a	Jun. 04, 2021 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Real Estate [Line Items]
Proceeds from sales of real estate				$ 67,821	$ 0	$ 81,871
Gains on sale of real estate				$ 53,952	$ 0	$ 69,950
Rego Park III Land Parcel
Real Estate [Line Items]
Proceeds from sales of real estate	$ 71,060
Net proceeds from sale of land	67,821
Gains on sale of real estate	$ 53,952
Bronx Land Parcel
Real Estate [Line Items]
Proceeds from sales of real estate			$ 9,291
Gains on sale of real estate			9,124
Proceeds from sale of land, excluding disposal cost			$ 10,000
IKEA \| Tenant Occupant \| Paramus
Real Estate [Line Items]
Net proceeds from sale of land		$ 4,580
Gains on sale of real estate		60,826
Proceeds from sale of land, excluding disposal cost		$ 75,000
Area of land \| a		30.3
Repayments of mortgage loan		$ 68,000