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Accumulated Other Comprehensive Loss - Summary of Accumulated Other Comprehensive Loss (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Pension liability adjustment, tax effect	$ 1,108nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit	$ 704nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit
Net deferred gain (loss) on cash flow derivatives, tax effect	$ 14nwpx_AccumulatedOtherComprehensiveIncomeLossCumulativeChangeInNetGainLossFromCashFlowHedgeEffectTax	$ 9nwpx_AccumulatedOtherComprehensiveIncomeLossCumulativeChangeInNetGainLossFromCashFlowHedgeEffectTax

X

- Definition

Tax expense (benefit) included in accumulated other comprehensive income for cash flow hedges.

+ References

No definition available.

+ Details

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Namespace Prefix:	nwpx_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Tax expense (benefit) included in accumulated other comprehensive income for defined benefit pension and other postretirement plans.

+ References

No definition available.

+ Details

Name:	nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit
Namespace Prefix:	nwpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration