Asset sale commission threshold.

No definition available.

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

- Definition

Percentage of development costs that the related party will receive as per the Development agreement.

+ References

No definition available.

+ Details

Name:	alx_DevelopmentFeePercentage
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease Commission, Percentage

+ References

No definition available.

+ Details

Name:	alx_LeaseCommissionPercentage
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the eleventh through twentieth years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentElevenToTwentyYears
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the first ten years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentOneToTenYears
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the twenty-first through thirtieth years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentTwentyFirstToThirtyYears
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of management fee paid to related party to manage corporate activities.

+ References

No definition available.

+ Details

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

- Definition

Percentage of gross proceeds over $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

Name:	alx_PercentageCommissionsOnSaleOfAssetsOverFiftyMillion
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of gross proceeds under $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

Name:	alx_PercentageCommissionsOnSaleOfAssetsUnderFiftyMillion
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of increase in leasing services payable to related party if a real estate broker is used

+ References

No definition available.

+ Details

Name:	alx_PercentageIncreaseLeaseFeeIfBrokerUsed
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Property Management Fee Agreement, Price Per Square Foot

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeAgreementPricePerSquareFoot
Namespace Prefix:	alx_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Annual increase in the amount paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeEscalationPercentagePerAnnum
Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of liabilities classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 480
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 64
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481620/480-10-55-64

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477548/944-605-55-14

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 55
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477548/944-605-55-11

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(17))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

Name:	us-gaap_OtherLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyManagementFeePercentFee
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

+ Details

Name:	us-gaap_RelatedPartyTransactionAmountsOfTransaction
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 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-13

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c)(2)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c)(2)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(g)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_PropertyManagementFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_LeasingFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_DevelopmentFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark2PropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_RelatedPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_ManagementAndDevelopmentAgreementBaseManagementFeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_ManagementPropertyManagementCleaningEngineeringAndSecurityFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=alx_AlexandersInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=alx_VornadoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions - Additional Information (Details)			3 Months Ended		6 Months Ended
	May 01, 2024	Apr. 30, 2024	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) $ / ft²	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Related Party Transaction [Line Items]
Fees to related party			$ 2,600,000	$ 7,541,000	$ 5,191,000	$ 9,930,000
Property Management Fees
Related Party Transaction [Line Items]
Fees to related party			1,464,000	1,213,000	2,923,000	2,849,000
Leasing fees
Related Party Transaction [Line Items]
Fees to related party			229,000	5,517,000	242,000	5,555,000
Development fees
Related Party Transaction [Line Items]
Fees to related party			207,000	$ 111,000	626,000	$ 126,000
Vornado
Related Party Transaction [Line Items]
Management fee agreement value					$ 2,800,000
Vornado \| Property Management Fees \| Rego Park II \| Retail Space
Related Party Transaction [Line Items]
Property management fee, percent fee					2.00%
Vornado \| Property Management Fees \| 731 Lexington Avenue \| Office and Retail Space
Related Party Transaction [Line Items]
Property management fee agreement, price per square foot \| $ / ft²					0.50
Vornado \| Property Management Fees \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Property management fee escalation percentage per annum					3.00%
Vornado \| Leasing fees
Related Party Transaction [Line Items]
Lease fee percentage of rent one to ten years					3.00%
Lease fee percentage of rent eleven to twenty years					2.00%
Lease fee percentage of rent twenty first to thirty years					1.00%
Percentage increase lease fee if broker used		1.00%
Percentage of third-party lease commission	33.00%
Percentage commissions on sale of assets under fifty million					3.00%
Asset sale commission threshold					$ 50,000,000
Percentage commissions on sale of assets over fifty million					1.00%
Vornado \| Development fees
Related Party Transaction [Line Items]
Development fee as percentage of development costs					6.00%
Related Party
Related Party Transaction [Line Items]
Other Liabilities			996,000		$ 996,000		$ 1,159,000
Related Party \| Management And Development Agreement, Base Management Fee \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Fees to related party					387,000
Related Party \| Leasing fees
Related Party Transaction [Line Items]
Other Liabilities			242,000		242,000		171,000
Related Party \| Development fees
Related Party Transaction [Line Items]
Other Liabilities			207,000		207,000		346,000
Related Party \| Management, property management, cleaning, engineering and security fees
Related Party Transaction [Line Items]
Other Liabilities			$ 547,000		$ 547,000		$ 642,000
Alexander's Inc. \| Vornado \| Vornado
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners			32.40%		32.40%