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Discontinued Operations (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 USD ($)
Discontinued Operations and Disposal Groups [Abstract] |
Total real property tax expense | $ 23,797
Additional real property transfer tax expense | 15,874
Real property tax expense interest | $ 7,923

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- Definition Additional Real Estate Transfer Tax + References No definition available. + Details
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Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Real Estate Tax Expense Interest + References No definition available. + Details
Name: | alx_RealEstateTaxExpenseInterest
Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_DiscontinuedOperationsAndDisposalGroupsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition A tax based on the assessed value of real estate by the local government. The tax is usually based on the value of property (including the land). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6419918&loc=d3e35301-107843 + Details
Name: | us-gaap_RealEstateTaxExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration