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Note 10 - Segment Information (Details Textual) - Operating Segments [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Engineered Steel Pressure Pipe [Member]
Depreciation, Depletion and Amortization, Nonproduction	$ 2.5	$ 2.7
Precast Infrastructure and Engineered Systems [Member]
Depreciation, Depletion and Amortization, Nonproduction	$ 0.8	$ 0.5

X

- Definition

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482099/360-10-50-1

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

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- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=nwpx_EngineeredSteelPressurePipeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
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Period Type: