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

Minimum development fee that Vornado will receive per annum, as per the development agreement

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

Maximum annual installments payable for leasing services and commission on asset sales under the leasing agreement.

No definition available.

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

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

No definition available.

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

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

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

The fee charged to supervise cleaning, engineering and security services

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

Annual amount paid to related party as property management fees

No definition available.

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

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

No definition available.

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

Interest rate charged on unpaid commissions as of at end of the current reporting period

No definition available.

Name:	alx_UnpaidBalanceEffectiveInterest
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

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

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

No definition available.

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

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

No definition available.

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

The percentage charged for managing real estate properties.

No definition available.

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

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.

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph k

-Subparagraph 1

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 57

-Paragraph 2

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: 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=6881521&loc=d3e23780-122690

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