No definition available.

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

- Definition

Lease expiration date in yyyy-mm format.

+ References

No definition available.

+ Details

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

- Definition

Approximate rental term range for all outstanding leases to tenants in retail centers and office buildings.

+ References

No definition available.

+ Details

Name:	alx_LeasesOfLessorRentalTermRange
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
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

Number Of Fires

+ References

No definition available.

+ Details

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

- References

No definition available.

+ 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

- 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

+ Details

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

- Definition

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 element is net of accumulated amortization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 17
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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

- 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

- References

No definition available.

+ Details

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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
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Name:	us-gaap_LossContingencyDamagesSoughtValue
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Paragraph 1
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Name:	us-gaap_OperatingLeasesIncomeStatementLeaseRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the adjustment to rental revenues to measure escalating leasing revenues on a straight line basis.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark1PropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_ParamusPropertyMember
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- Details

Name:	alx_RealEstateOccupantPossessionTypeAxis=alx_TenantOccupantMember
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- Details

Name:	alx_RealEstatePropertyOccupantAxis=alx_IkeaMember
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Commitments and Contingencies (Litigation and contingencies) (Details) ft² in Thousands, $ in Thousands			1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 15, 2015 USD ($)	Jun. 24, 2014 USD ($)	Feb. 28, 2014 Fires	Mar. 31, 2017 USD ($) ft²	Mar. 31, 2016 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2001 a
Loss Contingency
Property rentals				$ 38,273	$ 36,653
Deferred leasing cost, net				47,271			$ 48,387
Standby letters of credit, outstanding				2,074
Mortgages payable, net of deferred debt issuance costs				1,051,973			$ 1,052,359
Rego Park I \| Sears
Loss Contingency
Number Of Fires \| Fires			2
Property rentals				10,337
Deferred leasing cost, net				$ 499
Lease expiration date				March 2021
Straight line rent receivable				$ 4,757
Paramus Property
Area of property (in square feet) \| ft²				195
Rego Park I \| Sears \| Minimum
Loss Contingency
Loss Contingency, Damages Sought, Value		$ 4,000
Rego Park I \| Sears \| Estimated Future Damages
Loss Contingency
Loss Contingency, Damages Sought, Value				$ 650	$ 650
Rego Park I \| Sears \| Estimated Future Damages \| Minimum
Loss Contingency
Loss Contingency, Damages Sought, Value		$ 25,000
Paramus \| Tenant Occupant \| Ikea
Loss Contingency
Area Of Land \| a								30.3
Mortgages payable, net of deferred debt issuance costs				$ 68,000
Paramus Property
Fixed interest rate on the debt (in percentage)				2.90%
Debt instrument maturity date				October 2018
Triple-net rent, annual amount				$ 700
Paramus \| Tenant Occupant \| Ikea \| Forecast
Paramus Property
Property purchase option exercisable be leasee with purchase option in 2021						$ 75,000
Purchase option exercised, net cash proceeds from sale of land						7,000
Purchase option excercised, gain on sale of land						60,000
Purchase option not excercised amount included in triple net rent over remainder of lease						$ 68,000
Loan Amortization Period						20 years
Lease term range as Lessor						20 years
The Kings Plaza Regional Shopping Center
Loss Contingency
Loss Contingency, Damages Sought, Value	$ 21,700