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Multiemployer Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plans Period Contributions Significance Of Contributions | false |  |
Multiemployer Pension Plans [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | $ 144 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPensionMember | $ 138 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPensionMember | $ 196 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPensionMember
Multiemployer plan period contributions from discontinuing operations |  |  | 61 alx_MultiemployerPlanPeriodContributionsDiscontinuingOperations / us-gaap_MultiemployerPlanTypeAxis = us-gaap_MultiemployerPlansPensionMember
Multiemployer Health Plans [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | 533 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = alx_MultiemployerHealthPlansMember | 499 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = alx_MultiemployerHealthPlansMember | 734 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanTypeAxis = alx_MultiemployerHealthPlansMember
Multiemployer plan period contributions from discontinuing operations |  |  | $ 250 alx_MultiemployerPlanPeriodContributionsDiscontinuingOperations / us-gaap_MultiemployerPlanTypeAxis = alx_MultiemployerHealthPlansMember

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- Definition Multiemployer plan period contributions relating to discontinuing operations. + 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 Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450702-114947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450691-114947 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
Name: | us-gaap_MultiemployerPlanPeriodContributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition Description of whether the employer's contributions represent more than five percent of total contributions to the multiemployer plan for the most recent plan year available. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (e)(2) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:booleanItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_MultiemployerPlanTypeAxis=alx_MultiemployerHealthPlansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MultiemployerPlanTypeAxis=us-gaap_MultiemployerPlansPensionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |