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Multiemployer Benefit Plans (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
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 | $ 144 | $ 138
Multiemployer Health Plans [Member] |  |  |
Multiemployer Plans [Line Items] |  |  |
Multiemployer Plan, Period Contributions | $ 554 | $ 533 | $ 499

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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=49170332&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=49170332&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=49170332&loc=SL14450657-114947 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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=49170332&loc=SL14450657-114947 + Details
Name: | us-gaap_MultiemployerPlansPeriodContributionsSignificanceOfContributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:booleanItemType
Balance Type: | na
Period Type: | duration

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

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