Annual revenue attributable to tenant.

No definition available.

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

- Definition

Asset sale commission threshold.

+ References

No definition available.

+ Details

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:	num: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:	num: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:	num: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:	num: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:	num: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:	num: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:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Property Management Fee Agreement, Price Per Square Foot

+ References

No definition available.

+ Details

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

- Definition

Annual amount paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeAgreementValue
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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

- Definition

Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_RelatedPartyTransactionDueFromToRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- 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:	alx_RealEstatePropertyOccupantAxis=alx_ToysRUsMember
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:	us-gaap_RelatedPartyTransactionAxis=alx_PropertyManagementFeesMember
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_RelatedPartyTransactionAxis=alx_DevelopmentFeesMember
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_ManagementPropertyManagementCleaningEngineeringAndSecurityFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	dei_LegalEntityAxis=alx_VornadoMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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

Name:	srt_OwnershipAxis=alx_ToysRUsMember
Namespace Prefix:
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Related Party Transactions - Additional Information (Detail) ft² in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / ft²	Dec. 31, 2018 USD ($)	Jun. 30, 2018 ft²
Rego Park II \| Retail Space \| Toys R Us
Related Party Transaction [Line Items]
Area of property (in sqft.) \| ft²			47
Annual revenue from leased property		$ 2,600,000
Vornado
Related Party Transaction [Line Items]
Management fee agreement value	$ 2,800,000
Vornado \| Property management, cleaning, engineering and security fees \| Rego Park II \| Retail Space
Related Party Transaction [Line Items]
Property management fee agreement percentage of income	2.00%
Vornado \| Property management, cleaning, engineering and security 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, cleaning, engineering and security fees \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Property management fee agreement value	$ 324,000
Property management fee escalation percentage per annum	3.00%
Vornado \| Development fees
Related Party Transaction [Line Items]
Development fee as percentage of development costs	6.00%
Amounts due from / (Amounts due to) related party	$ (68,000)	(146,000)
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 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%
Amounts due from / (Amounts due to) related party	$ (3,871,000)	(13,000)
Vornado \| Management, property management, cleaning, engineering and security fees
Related Party Transaction [Line Items]
Amounts due from / (Amounts due to) related party	$ (699,000)	$ (549,000)
Vornado \| Alexander's Inc. \| Vornado
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	32.40%
Vornado \| Toys R Us \| Vornado
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by parent		32.50%