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v3.20.4

Commitments and Contingencies (Insurance) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($)
General Liability |
Insurance |
Insurance maximum coverage per incident | $ 300,000,000
Insurance maximum coverage per property | 300,000,000
Disease Coverage |
Insurance |
Insurance maximum coverage per property | 1,000,000
All Risk Property And Rental Value |
Insurance |
Insurance maximum coverage per incident | 1,700,000,000
Terrorism Coverage Including NBCR |
Insurance |
Insurance maximum coverage per incident | $ 1,700,000,000
Insurance coverage end date | Dec. 01, 2027
Insurance maximum coverage in aggregate | $ 1,700,000,000
NBCR |
Insurance |
Federal government responsibility (in percentage) | 80.00%
FNSIC | NBCR |
Insurance |
Insurance deductible | $ 275,000
Self insured responsibility (in percentage) | 20.00%

X
- 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

X
- Definition Insurance coverage end date. + References No definition available. + Details
Name: | alx_InsuranceCoverageEndDate
Namespace Prefix: | alx_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition The deductible owed by our wholly owned insurance subsidiary in the event of a covered loss. + References No definition available. + Details
Name: | alx_InsuranceDeductible
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Insurance maximum coverage limit in aggregate. + References No definition available. + Details
Name: | alx_InsuranceMaximumCoverageInAggregate
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- Definition Insurance Maximum Coverage Per Property. + References No definition available. + Details
Name: | alx_InsuranceMaximumCoveragePerProperty
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- References No definition available. + Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis=us-gaap_GeneralLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis=alx_DiseaseCoverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_AllRiskPropertyAndRentalValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_TerrorismCoverageIncludingNbcrMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | alx_ScheduleOfInsuranceTypeAndTierIdentifierAxis=alx_NbcrMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=alx_FiftyNinthStreetInsuranceCompanyLlcMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |