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Note 14 - Recent Accounting and Reporting Developments (Details Textual) - Reclassification from Cost of Sales to Other Income (Expense) [Member]	9 Months Ended
	Sep. 30, 2018 USD ($)
Three Months Ended September 30, 2017 [Member]
Prior Period Reclassification Adjustment	$ 0
Nine Months Ended September 30, 2017 [Member]
Prior Period Reclassification Adjustment	$ 200,000

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CreationDateAxis=nwpx_ThreeMonthsEndedSeptember302017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CreationDateAxis=nwpx_NineMonthsEndedSeptember302017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: