Debt instrument maturity date

No definition available.

Name:	alx_DebtInstrumentMaturityDateString
Namespace Prefix:	alx_
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The deductible owed by our wholly owned insurance subsidiary in the event of a covered loss

+ References

No definition available.

+ Details

Name:	alx_Deductible
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Estimated total cost for proposed development.

+ References

No definition available.

+ Details

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

- Definition

Estimated Date Of Completion of a real estate property.

+ References

No definition available.

+ Details

Name:	alx_EstimatedDateOfCompletion
Namespace Prefix:	alx_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of the covered portion of a loss above the deductible for which the Federal Government is responsible for covered losses

+ References

No definition available.

+ Details

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

- Definition

This element represents end date of insurance coverage.

+ References

No definition available.

+ Details

Name:	alx_InsuranceCoverageEndDate
Namespace Prefix:	alx_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Insurance maximum coverage limit per incident

+ References

No definition available.

+ Details

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

- Details

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

- Definition

In the event the tenant does not exercise its purchase option, the period over which the tenant's debt service portion of its rent must be sufficient to fully amortize the loan on the property

+ References

No definition available.

+ Details

Name:	alx_LoanAmortizationPeriod
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loan that will become recourse in case of a substantial casualty

+ References

No definition available.

+ Details

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

- Definition

Number Of Fires

+ References

No definition available.

+ Details

Name:	alx_NumberOfFires
Namespace Prefix:	alx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

Lease with purchase option to be exercised by lessee.

+ References

No definition available.

+ Details

Name:	alx_PropertyPurchaseOptionExcercisableByLessee
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash inflow from the sale of land held for use in case the purchase option is exercised by leasee.

+ References

No definition available.

+ Details

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

- Definition

The amount to be amortized over the remainder of the lease term if the purchase option is not exercised.

+ References

No definition available.

+ Details

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

- Details

Name:	alx_RegoPark2PropertyAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of the covered portion of a loss above the deductible for which our wholly owned insurance subsidiary is responsible for covered losses

+ References

No definition available.

+ Details

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

- Definition

Triple-net rent, annual amount

+ References

No definition available.

+ Details

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

- Definition

Area of land held.

+ References

No definition available.

+ Details

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

- 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

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The current amount of expenditures for a real estate project that has not yet been completed.

+ 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 210.7-03.10)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Amount of difference between the carrying amount and the sales price of real estate properties sold as part of discontinued operations.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

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

- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9

+ Details

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

- Definition

The number of units in a real estate property owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfUnitsInRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_AllRiskPropertyAndRentalValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_NbcrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_TerrorismCoverageIncludingNbcrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=us-gaap_GeneralLiabilityMember
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_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_ParamusPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark1ShoppingMember
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:

Commitments and Contingencies (Details) (USD $)	3 Months Ended	12 Months Ended	0 Months Ended
Commitments and Contingencies (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2021	Jun. 24, 2014 Fires	Dec. 31, 2014	Dec. 31, 2001 acre	Jan. 01, 2016
Rego Park 2 Property [Abstract]
Development and construction in progress	$ 89,244,000us-gaap_DevelopmentInProcess			$ 75,289,000us-gaap_DevelopmentInProcess
Paramus Property [Abstract]
Mortgages payable	1,032,004,000us-gaap_NotesPayable			1,032,780,000us-gaap_NotesPayable
Letters Of Credit [Abstract]
Standby letters of credit, issued and outstanding	2,074,000
Lexington Avenue Property [Member] \| Retail Space [Member]
Insurance [Abstract]
Mortgage loan, recourse	75,000,000
Paramus Property [Abstract]
Mortgages payable	320,000,000
Paramus Property [Member] \| Tenant Occupant [Member]
Paramus Property [Abstract]
Debt Instrument Maturity Date String	October 2018
Paramus Property [Member] \| Tenant Occupant [Member] \| Ikea [Member]
Paramus Property [Abstract]
Area Of Land (in acre)					30.3
Mortgages payable	68,000,000
Fixed interest rate on the debt (in percentage)	2.90%
Triple-net rent, annual amount	700,000
Paramus Property [Member] \| Tenant Occupant [Member] \| Ikea [Member] \| Scenario Forecast [Member]
Paramus Property [Abstract]
Property purchase option exercisable be leasee with purchase option in 2021		75,000,000
Purchase option exercised, net cash proceeds from sale of land		7,000,000
Purchase option excercised, gain on sale of land		60,000,000
Purchase option not excercised amount included in triple net rent over remainder of lease		68,000,000
Loan Amortization Period		20 years
Rego Park I [Member] \| Sears [Member]
Loss Contingency [Abstract]
Number Of Fires			2
Rego Park I [Member] \| Sears [Member] \| Minimum [Member]
Loss Contingency [Abstract]
Loss Contingency, Damages Sought, Value			4,000,000
Rego Park I [Member] \| Sears [Member] \| Minimum [Member] \| Estimated Future Damages [Member]
Loss Contingency [Abstract]
Loss Contingency, Damages Sought, Value			25,000,000
Rego Park 2 Property [Member] \| Apartment Tower [Member] \| Property Under Development [Member]
Rego Park 2 Property [Abstract]
Number of units in apartment tower (in units)	312
Area of property (in square feet)	255,000
Estimated Date Of Completion	2015
Estimated Budgeted Cost	125,000,000
Development and construction in progress	87,087,000
General Liability [Member]
Insurance [Abstract]
Insurance Maximum Coverage Per Incident	300,000,000
All Risk Property And Rental Value [Member]
Insurance [Abstract]
Insurance Maximum Coverage Per Incident	1,700,000,000
Terrorism Coverage Including Nbcr [Member]
Insurance [Abstract]
Insurance Maximum Coverage Per Incident	1,700,000,000
NBCR [Member]
Insurance [Abstract]
Insurance Coverage End Date	December 2020
Deductible	$ 275,000
Self Insured Responsibility (in percentage)	15.00%
Federal Government Responsibility (in percentage)	85.00%
NBCR [Member] \| Scenario Forecast [Member]
Insurance [Abstract]
Self Insured Responsibility (in percentage)						16.00%
Federal Government Responsibility (in percentage)						84.00%