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Note 10 - Commitments and Contingencies (Details Textual) $ in Millions	1 Months Ended
	Jan. 31, 2017 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2014 USD ($)
Other Commitment, Amount Paid for Equipment Purchased Yet Not Received		$ 7.3
Letters of Credit Outstanding, Amount		$ 1.1
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

X

- Definition

The estimated cost of the remedy selected by EPA.

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	nwpx_
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X

- Definition

Number of potentially responsible parties for Portland Harbor Superfund site.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of commitment of payment for equipment purchased but not received.

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

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Balance Type:	credit
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X

- Details

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Namespace Prefix:
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X

- Details

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Period Type:

X

- Details

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