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Commitments and Contingencies (Insurance) (Details) - USD ($) |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017
Insurance |  |
Insurance maximum coverage per incident |  | $ 300,000,000
Insurance maximum coverage per property |  | 300,000,000
All Risk Property And Rental Value |  |
Insurance |  |
Insurance maximum coverage per incident |  | 1,700,000,000.0
Terrorism Coverage Including NBCR |  |
Insurance |  |
Insurance maximum coverage per incident |  | $ 1,700,000,000.0
Insurance coverage end date |  | Dec. 01, 2020
Insurance maximum coverage in aggregate |  | $ 1,700,000,000.0
FNSIC | NBCR |  |
Insurance |  |
Insurance deductible |  | $ 293,000
Self Insured Responsibility (in percentage) |  | 17.00%
Federal Government Responsibility (in percentage) |  | 83.00%
FNSIC | NBCR | Subsequent Event |  |
Insurance |  |
Insurance deductible | $ 306,000 |
Self Insured Responsibility (in percentage) | 18.00% |
Federal Government Responsibility (in percentage) | 82.00% |

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
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Namespace Prefix: | alx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Insurance coverage end date. + References No definition available. + Details
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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
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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
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Data Type: | xbrli:monetaryItemType
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Period Type: | duration

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- Definition Insurance maximum coverage limit per incident. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |