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Marketable Securities (Details) $ in Thousands | 1 Months Ended
--- | ---
Dec. 31, 2021 USD ($) shares
Marketable Securities [Abstract] |
Number of shares sold (shares) | shares | 564,612
Proceeds from sale of marketable securities | $ | $ 9,506

X
- Definition Number of Shares Sold + References No definition available. + Details
Name: | alx_NumberOfSharesSold
Namespace Prefix: | alx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period. + References No definition available. + Details
Name: | us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration