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Note 18 - Accumulated Other Comprehensive Loss - Summary of Accumulated Other Comprehensive Loss (Details) (Parentheticals) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Pension liability adjustment, tax effect | $ 911 | $ 866
Unrealized gain (loss) on cash flow hedges, tax effect | $ 8 | $ (1)

X
- Definition Tax expense (benefit) included in accumulated other comprehensive income for cash flow hedges. + References No definition available. + Details
Name: | nwpx_AccumulatedOtherComprehensiveIncomeLossCumulativeChangeInNetGainLossFromCashFlowHedgeEffectTax
Namespace Prefix: | nwpx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Tax expense (benefit) included in accumulated other comprehensive income for defined benefit pension and other postretirement plans. + References No definition available. + Details
Name: | nwpx_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTaxExpenseBenefit
Namespace Prefix: | nwpx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration