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Accumulated Other Comprehensive Loss - Schedule of Reclassifications of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of Sales	$ (80,974)us-gaap_CostOfRevenue	$ (78,333)us-gaap_CostOfRevenue
Tax (expense) benefit	1,273us-gaap_IncomeTaxExpenseBenefit	667us-gaap_IncomeTaxExpenseBenefit
Net loss	(2,101)us-gaap_NetIncomeLoss	(12,104)us-gaap_NetIncomeLoss
Net sales	84,865us-gaap_SalesRevenueNet	82,647us-gaap_SalesRevenueNet
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net loss	23us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember	(21)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net loss	(53)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	(28)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Defined Benefit Pension Items [Member] | Net Periodic Pension Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of Sales	(85)us-gaap_CostOfRevenue / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_PensionCostsMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	(43)us-gaap_CostOfRevenue / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_PensionCostsMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Defined Benefit Pension Items [Member] | Associated Tax Expense Benefit [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (expense) benefit	32us-gaap_IncomeTaxExpenseBenefit / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = nwpx_IncomeTaxExpenseBenefitMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	15us-gaap_IncomeTaxExpenseBenefit / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = nwpx_IncomeTaxExpenseBenefitMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Gains (Losses) on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net loss	76us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember	7us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Gains (Losses) on Cash Flow Hedges [Member] | Associated Tax Expense Benefit [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (expense) benefit	(45)us-gaap_IncomeTaxExpenseBenefit / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = nwpx_IncomeTaxExpenseBenefitMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember	(4)us-gaap_IncomeTaxExpenseBenefit / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = nwpx_IncomeTaxExpenseBenefitMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Gains (Losses) on Cash Flow Hedges [Member] | Gain on Cash Flow Derivative [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net sales	122us-gaap_SalesRevenueNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_GainLossOnDerivativeInstrumentsMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember	11us-gaap_SalesRevenueNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_GainLossOnDerivativeInstrumentsMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) [Member] | Gains (Losses) on Cash Flow Hedges [Member] | Hedge ineffectiveness [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net sales	$ (1)us-gaap_SalesRevenueNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_CashFlowHedgingMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember

X

- Definition

The aggregate cost of goods produced and sold and services rendered during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2)

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Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

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-Paragraph 1

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-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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-Section 50

-Paragraph 9

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

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-Section 45

-Paragraph 28

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

-Section 04

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X

- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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