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Leases - Schedule of Future Minimum Lease Payments for Capital Leases (Parenthetical) (Detail) | Dec. 31, 2014
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Leases [Abstract] |
Weighted average interest rate on capital leases | 9.66%nwpx_CapitalLeaseWeightedAverageInterestRate

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- Definition The weighted average interest rate on capital leases. + References No definition available. + Details
Name: | nwpx_CapitalLeaseWeightedAverageInterestRate
Namespace Prefix: | nwpx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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