Note 8 - Long-term Debt (Details Textual) - USD ($) $ in Millions		12 Months Ended
Oct. 28, 2024	Dec. 31, 2023	Dec. 31, 2024
Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration]	Secured Overnight Financing Rate (SOFR) [Member]	Secured Overnight Financing Rate (SOFR) [Member]
Wells Fargo Bank, N.A. [Member] \| Term Loan [Member]
Debt Instrument, Basis Spread on Variable Rate	2.22%
Debt Instrument, Periodic Payment	$ 0.3
Long-Term Debt			$ 14.5
Debt, Weighted Average Interest Rate			6.90%
Debt Instrument, Prepayment Fee, Percentage of Outstanding Principal	2.00%
Conversion of Interim Funding Agreement Debt into Term Loan [Member] \| Wells Fargo Bank, N.A. [Member]
Debt Conversion, Converted Instrument, Amount	$ 15.0

The prepayment fee, expressed as a percentage of the outstanding principal under debt instrument.

No definition available.

Name:	nwpx_DebtInstrumentPrepaymentFeePercentageOfOutstandingPrincipal
Namespace Prefix:	nwpx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The value of the financial instrument(s) that the original debt is being converted into in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
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+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the required periodic payments including both interest and principal payments.

+ References

+ Details

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

- Definition

Indicates type of variable interest rate on debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
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Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References

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-Name Accounting Standards Codification
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+ Details

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

- Details

Name:	srt_CounterpartyNameAxis=nwpx_WellsFargoBankNAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwpx_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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