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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Subsequent Events (Details) $ in Thousands | Sep. 30, 2015 USD ($)
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American Galvanizing |
Subsequent Event [Line Items] |
Annual sales | $ 10,000

X
- Definition Total revenue reported by the acquired entity for its previous full fiscal year before the business acquisition. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionRevenueReportedByAcquiredEntityForLastAnnualPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=vmi_AmericanGalvanizingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |