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Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Effective Income Tax Rate Reconciliation
Provision (benefit) at statutory rate of 35%	$ (532)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 11,921us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 9,083us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State provision, net of federal benefit	(96)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	370us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	601us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Federal and state tax credits	(91)nwpx_IncomeTaxReconciliationTaxFederalAndStateTaxCredits	(525)nwpx_IncomeTaxReconciliationTaxFederalAndStateTaxCredits	(379)nwpx_IncomeTaxReconciliationTaxFederalAndStateTaxCredits
Domestic manufacturing deduction		(641)us-gaap_IncomeTaxReconciliationDeductionsQualifiedProductionActivities	(762)us-gaap_IncomeTaxReconciliationDeductionsQualifiedProductionActivities
Change in valuation allowance	9us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	954us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Uncertain tax positions	5us-gaap_IncomeTaxReconciliationDeductions	(7)us-gaap_IncomeTaxReconciliationDeductions	(67)us-gaap_IncomeTaxReconciliationDeductions
Goodwill impairment (nondeductible)	5,623us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses
Nondeductible expenses	207us-gaap_IncomeTaxReconciliationNondeductibleExpense	345us-gaap_IncomeTaxReconciliationNondeductibleExpense	247us-gaap_IncomeTaxReconciliationNondeductibleExpense
Nontaxable adjustment to contingent consideration	(611)us-gaap_IncomeTaxReconciliationTaxExemptIncome
Other	137us-gaap_IncomeTaxReconciliationOtherAdjustments	(59)us-gaap_IncomeTaxReconciliationOtherAdjustments	(335)us-gaap_IncomeTaxReconciliationOtherAdjustments
Total	$ 4,651us-gaap_IncomeTaxExpenseBenefit	$ 12,358us-gaap_IncomeTaxExpenseBenefit	$ 8,388us-gaap_IncomeTaxExpenseBenefit
Effective tax rate	305.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations	36.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations	32.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations

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Income Tax Reconciliation Tax Federal And State Tax Credits

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

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

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible impairment loss.

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