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Retirement Plans - Additional Information (Detail) (USD $) | 12 Months Ended
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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 | 25 nwpx_DefinedContributionPlanNumberOfInvestmentOptions |  |
Number of noncontributory defined benefit plans | 2 nwpx_NumberOfNonContributoryDefinedBenefitPlans |  |
Accrued pension liability | $ 2,100,000 us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent | $ 1,200,000 us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent |
Unrecognized actuarial loss, net of tax | 1,900,000 nwpx_AccumulatedOtherComprehensiveIncomeLossUnrecognizedNetActuarialLossesNetOfTax | 1,300,000 nwpx_AccumulatedOtherComprehensiveIncomeLossUnrecognizedNetActuarialLossesNetOfTax |
Projected and accumulated benefit obligation | 6,900,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation | 5,800,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation |
Fair value of plan assets | 4,800,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets | 4,600,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
Net periodic benefit cost | 200,000 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | 400,000 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | 400,000 us-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% | 10,000 nwpx_OfficerContributionForWhichCompanyWillMatchAtFiftyPercent |  |
Non-qualified retirement savings plan, Select employee contributions for which Company will match at 50% | 5,000 nwpx_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 | $ 1,800,000 nwpx_RetirementPlanExpense | $ 1,800,000 nwpx_RetirementPlanExpense | $ 1,700,000 nwpx_RetirementPlanExpense

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- Definition Unrecognized actuarial loss recognized in accumulated other comprehensive loss, net of tax. + References No definition available. + Details
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- Definition Company matching percentage on first 6% of employee contributions under defined contribution retirement plan. + References No definition available. + Details
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- Definition Company matching percentage on first $10,000 of employee contributions under non-qualified retirement savings plan. + References No definition available. + Details
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- Definition Defined Contribution Plan Number Of Investment Options + References No definition available. + Details
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- Definition Employee contribution for which company will match at 50% under defined contribution retirement plan. + References No definition available. + Details
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- Definition Non-qualified retirement savings plan actuarial yearly asset growth percentage assumption. + References No definition available. + Details
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- Definition Non-qualified retirement savings plan target benefit amount. + References No definition available. + Details
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- Definition Number of noncontributory defined benefit plans. + References No definition available. + Details
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- Definition Officer contributions for which Company will match at 50% under non-qualified retirement savings plan. + References No definition available. + Details
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- Definition Retirement plan expense. + References No definition available. + Details
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- Definition Select employee contributions for which Company will match at 50% under non-qualified retirement savings plan. + References No definition available. + Details
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- Details
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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition This represents the entire liability recognized in the balance sheet that is associated with the defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2410-114920 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
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