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

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

No definition available.

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