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 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

- References

No definition available.

+ Details

Name:	alx_LeasingAgreementAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
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 Development Fee Escalation Percentage Per Annum

+ References

No definition available.

+ Details

Name:	alx_PropertyDevelopmentFeeEscalationPercentagePerAnnum
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount per square foot paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeAgreementPricePerSquareFoot
Namespace Prefix:	alx_
Data Type:	us-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

For an unclassified balance sheet, this element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This amount is before considering accumulated amortization representing the periodic charge to earnings to recognize the deferred costs over the term of the related lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40588-112709

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40246-112709

+ Details

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

- Definition

The cumulative difference between the rental payments required by a lease agreement and the rental income or expense recognized on a straight-line basis, or other systematic and rational basis more representative of the time pattern in which use or benefit is granted or derived from the leased property, expected to be recognized in income or expense over the term of the leased property, by the lessor or lessee, respectively. Such receivable is reduced by allowances attributable to, for instance, credit risk associated with a lessee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 35
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=77913719&loc=d3e40879-112712

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e39927-112707

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e39896-112707

+ Details

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

- Definition

Amount of commissions expense incurred because the lessor of real estate obtained a lessee for a rental property through a real estate agent.

+ References

No definition available.

+ Details

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

- 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

- References

No definition available.

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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

- Definition

Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Details

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

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark2PropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=us-gaap_RetailSiteMember
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_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_AlexanderApartmentTowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_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:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_LexingtonAvenuePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=alx_LeaseAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CounterpartyNameAxis=alx_ThirdPartyBrokerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions (Details) ft² in Thousands	1 Months Ended	12 Months Ended
Related Party Transactions (Details) ft² in Thousands	Jan. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) ft² $ / ft²	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Leasing Agreements
Receivable arising from the straight-lining of rents		$ 174,713,000	$ 179,010,000
Toys R Us/ Babies R Us
Leasing Agreements
Tenant improvements		694,000
Deferred leasing costs		257,000
Receivable arising from the straight-lining of rents		$ 544,000
Rego Park II \| Retail space \| Toys R Us/ Babies R Us
Leasing Agreements
Area of property (in square feet) \| ft²		47
Annual revenue attributable to tenant		$ 2,600,000
Development fees \| Alexander apartment tower
Leasing Agreements
Fees paid				$ 5,784,000
Vornado
Related Party Transaction
Noncontrolling interest, ownership percentage by parent		32.40%
Management and Development Agreements
Management fee agreement value		$ 2,800,000
Vornado \| Toys R Us/ Babies R Us
Related Party Transaction
Noncontrolling interest, ownership percentage by parent		32.50%
Vornado \| Property Management Fees \| Rego Park II
Management and Development Agreements
Property management fee, percent fee		2.00%
Vornado \| Property Management Fees \| 731 Lexington Avenue
Management and Development Agreements
Property management fee agreement price per square foot \| $ / ft²		0.50
Vornado \| Property Management Fees \| 731 Lexington Avenue \| Common Area
Management and Development Agreements
Property management fee agreement value		$ 306,000
Property management fee escalation percentage per annum		3.00%
Vornado \| Development fees
Leasing Agreements
Fees paid		$ 21,000	54,000
Vornado \| Development fees \| Alexander apartment tower
Management and Development Agreements
Property development fee escalation percentage per annum		6.00%
Vornado \| Leasing fees
Leasing Agreements
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%
Fees paid		$ 1,811,000	415,000
Vornado \| Management, property management, cleaning and security fees
Leasing Agreements
Fees paid		$ 658,000	$ 428,000
Mr. Roth, Interstate, David Mandelbaum, and Russell B. Wight, Jr.
Related Party Transaction
Noncontrolling interest, ownership percentage by parent		26.20%
IP & Partners Through Vornado
Related Party Transaction
Noncontrolling interest, ownership percentage by parent		2.30%
Lease Amendment \| Bloomberg
Leasing Agreements
Leasing commissions expense	$ 8,916,000
Lease Amendment \| Bloomberg \| Vornado
Leasing Agreements
Leasing commissions expense	1,716,000
Third Party Broker \| Lease Amendment \| Bloomberg
Leasing Agreements
Leasing commissions expense	$ 7,200,000