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Retirement Plans - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
OptionPlan
Investment_Options
Dec. 31, 2013
Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]      
Defined contribution retirement plan, Company match on first 6% of employee contributions 50.00%nwpx_CompanyMatchingPercentageOnFirstSixPercentOfEmployeeContributions    
Defined contribution retirement plan, employee contributions for which Company will match at 50% 6.00%nwpx_EmployeeContributionPercentageForWhichCompanyWillMatchAtFiftyPercent    
Defined contribution retirement plan, number of investment options offered 25nwpx_DefinedContributionPlanNumberOfInvestmentOptions    
Number of noncontributory defined benefit plans 2nwpx_NumberOfNonContributoryDefinedBenefitPlans    
Accrued pension liability $ 2,100,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent $ 1,200,000us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent  
Unrecognized actuarial loss, net of tax 1,900,000nwpx_AccumulatedOtherComprehensiveIncomeLossUnrecognizedNetActuarialLossesNetOfTax 1,300,000nwpx_AccumulatedOtherComprehensiveIncomeLossUnrecognizedNetActuarialLossesNetOfTax  
Projected and accumulated benefit obligation 6,900,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation 5,800,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation  
Fair value of plan assets 4,800,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 4,600,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Net periodic benefit cost 200,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 400,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 400,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Weighted average discount rates used to measure projected benefit obligation 3.54%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.40%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate  
Expected weighted average long term rate of return on plan assets 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets  
Non-qualified retirement savings plan, Company match percentage on first $10,000 of employee contributions 50.00%nwpx_CompanyMatchingPercentageOnFirstTenThousandDollarEmployeeContributions    
Non-qualified retirement savings plan, Officer contributions for which Company will match at 50% 10,000nwpx_OfficerContributionForWhichCompanyWillMatchAtFiftyPercent    
Non-qualified retirement savings plan, Select employee contributions for which Company will match at 50% 5,000nwpx_SelectEmployeeContributionForWhichCompanyWillMatchAtFiftyPercent    
Non-qualified retirement savings plan target benefit amount 35% of final base pay after a 35-year career with the Company or 1% of final base pay per year of service.    
Non-qualified retirement savings plan actuarial yearly asset growth percentage assumption 8.00%nwpx_NonQualifiedRetirementSavingsPlanActuarialYearlyAssetGrowthPercentageAssumption    
Total expense for all retirement plans $ 1,800,000nwpx_RetirementPlanExpense $ 1,800,000nwpx_RetirementPlanExpense $ 1,700,000nwpx_RetirementPlanExpense