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Note 7 - Commitments and Contingencies (Details Textual) $ in Millions | 1 Months Ended |  |
--- | --- | --- | ---
Jan. 31, 2017 USD ($) | Mar. 31, 2024 USD ($) | Jun. 30, 2014 USD ($)
Letters of Credit Outstanding, Amount |  | $ 1.1 |
Capital Addition Purchase Commitments [Member] |  |  |
Other Commitment, Amount Paid for Equipment Purchased Yet Not Received |  | 1.1 |
Other Commitment |  | $ 4.6 |
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 |  |

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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Period Type: | instant

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 -Topic 450 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147483076/450-20-50-1 + Details
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X
- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

X
- Details
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Namespace Prefix: |
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X
- Details
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Namespace Prefix: |
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X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |