The estimated cost of the remedy selected by EPA.

No definition available.

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Namespace Prefix:	nwpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The estimated amount of time to complete the selected EPA remedy.

+ References

No definition available.

+ Details

Name:	nwpx_EstimatedTimeToCompleteSelectedEPARemedy
Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
Balance Type:	na
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- Definition

Number of potentially responsible parties for Portland Harbor Superfund site.

+ References

No definition available.

+ Details

Name:	nwpx_NumberOfPotentiallyResponsibleParties
Namespace Prefix:	nwpx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loss contingency liability expected to be resolved within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

+ Details

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

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=nwpx_PortlandHarborNaturalreSourcesTrusteeCouncilMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EnvironmentalRemediationSiteAxis=nwpx_PortlandHarborSuperfundSiteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=nwpx_LowerWillametteGroupMember
Namespace Prefix:
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Balance Type:
Period Type:

Note 10 - Commitments and Contingencies (Details Textual) $ in Millions	1 Months Ended
	Jan. 31, 2017 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2014 USD ($)
Letters of Credit Outstanding, Amount		$ 1.6
Portland Harbor Natural Resources Trustee Council [Member]
Loss Contingency, Accrual, Current			$ 0.4
Portland Harbor Superfund Site [Member]
Number Of Potentially Responsible Parties	150
Estimated Cost of EPA Selected Remedy	$ 1,000.0
Estimated Time to Complete Selected EPA Remedy (Year)	13 years
Lower Willamette Group [Member]
Number Of Potentially Responsible Parties	14