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Mortgages Payable (Details) $ in Thousands | Dec. 31, 2022 USD ($)
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Debt Disclosure [Abstract] |
Net carrying value of real estate collateralizing the debt | $ 614,245

X
- Definition The carrying value, as of the date of each statement of financial position presented, of real estate which is owned and pledged as collateral for related debt obligation + References No definition available. + Details
Name: | alx_PledgedAssetsSeperatelyReportedRealEstatePledgedAsCollateralAtCarryingValue
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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