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Share-Based Compensation Plans - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 OptionPlan | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of inactive stock option plan | 1 nwpx_NumberOfInactiveStockOptionPlans |  |
Number of active stock incentive plan | 1 nwpx_NumberOfActiveStockIncentivePlans |  |
Maximum duration over which awards become exercisable under the plan | 60 months |  |
Termination date of the option from date of grant, in years | 10 years |  |
Number of common stock shares available for future issuance | 643,393 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance |  |
Total intrinsic value of options exercised | $ 43,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Number of options granted | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
Minimum performance awards issued multiplier | 0.00%nwpx_MinimumPerformanceAwardsIssuedMultiplier |  |
Maximum performance awards issued multiplier | 200.00%nwpx_MaximumPerformanceAwardsIssuedMultiplier |  |
Directors [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock awards granted to non-employee directors | 9,150 us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember | 4,912 us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember | 4,807 us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember
Directors [Member] | Minimum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Grant date fair market value of stock awards | $ 36.00 nwpx_ShareBasedCompensationArrangementByStockBasedPaymentAwardGrantDateFairValue / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember | $ 27.49 nwpx_ShareBasedCompensationArrangementByStockBasedPaymentAwardGrantDateFairValue / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember | $ 23.40 nwpx_ShareBasedCompensationArrangementByStockBasedPaymentAwardGrantDateFairValue / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember
Directors [Member] | Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Grant date fair market value of stock awards | $ 36.41 nwpx_ShareBasedCompensationArrangementByStockBasedPaymentAwardGrantDateFairValue / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  |
Restricted Stock Units and Performance Stock Awards [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Maximum duration over which awards become exercisable under the plan | 3 years |  |
Unrecognized compensation expense | 4,100,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember |  |
Expected weighted average period of compensation expense to be recognized | 1 year 8 months 12 days |  |
Fair value of RSUs and PSAs vested | $ 3,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember | $ 2,100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember | $ 600,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember
PSAs [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Maximum duration over which awards become exercisable under the plan | 3 years |  |
PSAs at target level of performance included in unvested balance | 185,000 nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberAtTargetLevelOfPerformance / us-gaap_AwardTypeAxis = nwpx_PerformanceShareAwardsMember |  |
PSAs [Member] | Minimum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Maximum duration over which awards become exercisable under the plan | 2 years |  |
PSAs [Member] | Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Maximum duration over which awards become exercisable under the plan | 3 years |  |

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- Definition Maximum performance awards issued multiplier. + References No definition available. + Details
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- Definition Minimum performance awards issued multiplier. + References No definition available. + Details
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- Definition Number of active stock incentive plans. + References No definition available. + Details
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- Definition Number of inactive stock option plans. + References No definition available. + Details
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- Definition The number of non-vested equity-based payment instruments at a target level of performance, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award options award expiration period. + References No definition available. + Details
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- Definition Share based compensation arrangement by stock based payment award grant date fair value. + References No definition available. + Details
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- Definition Aggregate number of common shares reserved for future issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation. + References No definition available. + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares or units of entity securities issued in the transaction in which equity securities were issued to pay for goods or nonemployee services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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