XML 57 R46.htm IDEA: XBRL DOCUMENT v3.8.0.1

Note 12 - Recent Accounting and Reporting Developments (Details Textual) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Reclassification from Cost of Sales to Other Income (Expense) [Member] | Three Months Ended March 31, 2017 [Member]
Prior Period Reclassification Adjustment	$ 0.1

X

- Definition

The amount of a reclassification adjustment made to prior period financial statement amounts.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis=nwpx_ReclassificationFromCostOfSalesToOtherIncomeExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CreationDateAxis=nwpx_ThreeMonthsEndedMarch312017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: