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Shareholder Rights Plan - Additional Information (Detail) (USD $) | 0 Months Ended |
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Jun. 18, 2009 | Dec. 31, 2014 Right
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |
Number of series A junior participating preferred stock shares reserved |  | 150,000 nwpx_NumberOfSeriesJuniorParticipatingPreferredStockSharesReserved
Number of non-detachable preferred stock purchase right distributed as dividend to shareholders |  | 1 nwpx_DividendDistributionUnderShareholderRightsPlan
Right to purchase a share of Preferred Stock per each right |  | 0.01 us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
Price at which shareholders holding a right may purchase 1/100th of a share of preferred stock |  | $ 83.00 nwpx_PurchasePriceUnderRights
Acquisition percentage to cause rights to become exercisable |  | 15.00%nwpx_AcquisitionPercentage
Market value, multiple over exercise price |  | 2 nwpx_ValueOfExercisedRight
Value at which Company may redeem rights |  | $ 0.01 nwpx_RightRedemptionValue
Rights agreement expiration date | Jun. 28, 2019 |

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- Definition Percentage of Company's outstanding shares of common stock which person or group must acquire or commence a tender offer to acquire to cause Rights to become exercisable. + References No definition available. + Details
Name: | nwpx_AcquisitionPercentage
Namespace Prefix: | nwpx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Amended And Restated Rights Agreement Expiration Date + References No definition available. + Details
Name: | nwpx_AmendedAndRestatedRightsAgreementExpirationDate
Namespace Prefix: | nwpx_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Dividend distribution under shareholder rights plan. + References No definition available. + Details
Name: | nwpx_DividendDistributionUnderShareholderRightsPlan
Namespace Prefix: | nwpx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Series A Junior Participating Preferred Stock Shares reserved under the Shareholder Rights Plan. + References No definition available. + Details
Name: | nwpx_NumberOfSeriesJuniorParticipatingPreferredStockSharesReserved
Namespace Prefix: | nwpx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Price at which shareholders holding a Right may purchase 1/100th of a share of preferred stock under the Shareholder Rights Plan. + References No definition available. + Details
Name: | nwpx_PurchasePriceUnderRights
Namespace Prefix: | nwpx_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Under certain circumstances, the value at which the Company may redeem the right. + References No definition available. + Details
Name: | nwpx_RightRedemptionValue
Namespace Prefix: | nwpx_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition If exercised, the value of common stock of the Company or an acquiring company a Right holder may purchase. This amount is equal to two times the exercise price of the Right. + References No definition available. + Details
Name: | nwpx_ValueOfExercisedRight
Namespace Prefix: | nwpx_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of securities into which each warrant or right may be converted. For example, but not limited to, each warrant may be converted into two shares. + References No definition available. + Details
Name: | us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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