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Leases - Schedule of Future Minimum Lease Payments for Capital Leases (Parenthetical) (Detail)	Dec. 31, 2014
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