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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 | 2 us-gaap_NumberOfReportableSegments |  |  |  |  |
Carrying value of investment | $ 0 us-gaap_Investments | $ 0 us-gaap_Investments |  |  |  |
Loss on sale of discontinued operations | (13,497,000)us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax |  |  |  |  |
Goodwill written off related to sale of business | 4,412,000 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit |  |  |  |  |
Proceeds from sale of business | 29,791,000 us-gaap_ProceedsFromDivestitureOfBusinesses |  |  |  |  |
Business acquisition (net of received), amount paid |  | 15,689,000 us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired |  |  |  |
Business acquisition, goodwill |  | 5,282,000 us-gaap_GoodwillAcquiredDuringPeriod |  |  |  |
Book overdraft | 900,000 nwpx_BookOverdrafts | 0 nwpx_BookOverdrafts |  |  |  |
Advanced deposits | 0 us-gaap_CustomerAdvancesCurrent | 0 us-gaap_CustomerAdvancesCurrent |  |  |  |
Accrued vacation payable | 2,200,000 us-gaap_AccruedVacationCurrent | 2,200,000 us-gaap_AccruedVacationCurrent |  |  |  |
Number of defined benefit pension plans | 2 nwpx_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 | 1 nwpx_NumberOfCustomersWithReceivableBalanceInExcessOfTenPercentOfReceivables | 2 nwpx_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 |  |  | 15,700,000 us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_BusinessAcquisitionAxis = nwpx_PermalokCorporationMember |  |  |
Business acquisition, goodwill |  |  | 5,300,000 us-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 |  |  |  | 42,700,000 us-gaap_ProceedsFromSaleOfProductiveAssets / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |
Loss on sale of discontinued operations |  |  |  | 13,500,000 us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember | 1,800,000 us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |
Proceeds held in escrow for indemnification obligations under the purchase agreement |  |  |  | 4,300,000 nwpx_BusinessAcquisitionPurchasePriceAmountHeldInEscrowToSecureIndemnificationObligations / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |
Payment of capital lease obligations at closing date |  |  |  | 5,000,000 us-gaap_RepaymentsOfDebtAndCapitalLeaseObligations / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |
Goodwill written off related to sale of business | 4,400,000 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |  | 4,400,000 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |
Transaction costs |  |  |  | 1,800,000 us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |
Proceeds from sale of business | 29,800,000 us-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 | 2,700,000 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember / us-gaap_BusinessAcquisitionAxis = nwpx_PermalokCorporationMember | 4,400,000 us-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 |  |  |  |  |  | $ 0 us-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 |  |  |  |  |

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