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Share-Based Compensation Plans - Summary of Status of Company's RSUs and PSAs (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
vesting period | 60 months
Restricted Stock Units and Performance Stock Awards [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Target level of RSUs and PSAs | 100.00%nwpx_RestrictedStockUnitsAndPerformanceShareAwardTargetLevelPercentage / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember
vesting period | 3 years
PSAs [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Additional shares vested for awards | 13,161 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares / us-gaap_AwardTypeAxis = nwpx_PerformanceShareAwardsMember
vesting period | 3 years
Actual payout percentage | 143.00%nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForGrantActualPayoutPercentage / us-gaap_AwardTypeAxis = nwpx_PerformanceShareAwardsMember

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- Definition Restricted Stock Units And Performance Share Award Target Level Percentage + References No definition available. + Details
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- Definition Share Based Compensation Arrangement By Share Based Payment Award Shares Available For Grant Actual Payout Percentage + References No definition available. + Details
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Namespace Prefix: | nwpx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of options vested. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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

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