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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
 12 Months Ended 0 Months Ended 1 Months Ended
Dec. 31, 2014
Customer
Segment
Pension_Plan
Dec. 31, 2013
Customer
Dec. 30, 2013
Mar. 30, 2014
Sep. 30, 2014
Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]            
Number of reportable segments 2us-gaap_NumberOfReportableSegments          
Carrying value of investment $ 0us-gaap_Investments $ 0us-gaap_Investments        
Loss on sale of discontinued operations (13,497,000)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax          
Goodwill written off related to sale of business 4,412,000us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit          
Proceeds from sale of business 29,791,000us-gaap_ProceedsFromDivestitureOfBusinesses          
Business acquisition (net of received), amount paid   15,689,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired        
Business acquisition, goodwill   5,282,000us-gaap_GoodwillAcquiredDuringPeriod        
Book overdraft 900,000nwpx_BookOverdrafts 0nwpx_BookOverdrafts        
Advanced deposits 0us-gaap_CustomerAdvancesCurrent 0us-gaap_CustomerAdvancesCurrent        
Accrued vacation payable 2,200,000us-gaap_AccruedVacationCurrent 2,200,000us-gaap_AccruedVacationCurrent        
Number of defined benefit pension plans 2nwpx_NumberOfDefinedBenefitPensionPlans          
Change in gross profit which triggers senior management review 2.00%nwpx_ChangeInGrossProfitWhichTriggersManagementReview          
Minimum likelihood of tax benefit being realized in order for tax benefit to be recorded 50.00%nwpx_MinimumPercentageOfLikelihoodOfTaxBenefitBeingRealizedUponSettlement          
Number of customers with balances in excess of 10% of total accounts receivable 1nwpx_NumberOfCustomersWithReceivableBalanceInExcessOfTenPercentOfReceivables 2nwpx_NumberOfCustomersWithReceivableBalanceInExcessOfTenPercentOfReceivables        
Permalok Corporation [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Business acquisition date Dec. 30, 2013          
Business acquisition, percentage     100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember		      
Business acquisition (net of received), amount paid     15,700,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember		      
Business acquisition, goodwill     5,300,000us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember		      
Disposal of OCTG Business [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Business disposal date Mar. 30, 2014          
Proceeds from sale of business       42,700,000us-gaap_ProceedsFromSaleOfProductiveAssets
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		    
Loss on sale of discontinued operations       13,500,000us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		 1,800,000us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		  
Proceeds held in escrow for indemnification obligations under the purchase agreement       4,300,000nwpx_BusinessAcquisitionPurchasePriceAmountHeldInEscrowToSecureIndemnificationObligations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		    
Payment of capital lease obligations at closing date       5,000,000us-gaap_RepaymentsOfDebtAndCapitalLeaseObligations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		    
Goodwill written off related to sale of business 4,400,000us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		     4,400,000us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		    
Transaction costs       1,800,000us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		    
Proceeds from sale of business 29,800,000us-gaap_ProceedsFromDivestitureOfBusinesses
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= nwpx_OilCountryTubularGoodsMember		          
Other Long-term Liabilities [Member] | Permalok Corporation [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Business acquisition, contingent consideration recorded 2,700,000us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentLiabilitiesMember
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember		 4,400,000us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentLiabilitiesMember
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember		        
Scenario, Forecast [Member] | Permalok Corporation [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Business acquisition, contingent consideration recorded           $ 0us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nwpx_PermalokCorporationMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Minimum [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of intangible assets 3 years          
Minimum [Member] | Land Improvements [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 15 years          
Minimum [Member] | Buildings [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 20 years          
Minimum [Member] | Machinery and Equipment [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 3 years          
Maximum [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of intangible assets 15 years          
Maximum [Member] | Land Improvements [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 30 years          
Maximum [Member] | Buildings [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 40 years          
Maximum [Member] | Machinery and Equipment [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Estimated useful lives of property and equipment 30 years