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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
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Dec. 31, 2014 | Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] |  |
Pension liability adjustment, tax effect | $ 1,108 nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit | $ 704 nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit
Net deferred gain (loss) on cash flow derivatives, tax effect | $ 14 nwpx_AccumulatedOtherComprehensiveIncomeLossCumulativeChangeInNetGainLossFromCashFlowHedgeEffectTax | $ 9 nwpx_AccumulatedOtherComprehensiveIncomeLossCumulativeChangeInNetGainLossFromCashFlowHedgeEffectTax

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- Definition Tax expense (benefit) included in accumulated other comprehensive income for cash flow hedges. + References No definition available. + Details
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- Definition Tax expense (benefit) included in accumulated other comprehensive income for defined benefit pension and other postretirement plans. + References No definition available. + Details
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- Details
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Namespace Prefix: | us-gaap_
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