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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, 2019	Dec. 31, 2018
Pension liability adjustment, tax effect	$ 671	$ 911
Unrealized gain (loss) on cash flow hedges, tax effect	$ (11)	$ 8

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