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Note 20 - Quarterly Data (Unaudited) (Details Textual) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Gain (Loss) on Disposition of Property Plant Equipment |  |  | $ 51,000 | $ (19,000) | $ (105,000)
Denver Facility [Member] | Water Transmission Group [Member] |  |  |  |  |
Gain (Loss) on Disposition of Property Plant Equipment |  | $ 7,900,000 |  |  |
Cost of Sales [Member] |  |  |  |  |
Workers Compensation Expense | $ 1,200,000 |  |  |  |

X
- Definition Amount of workers compensation charge related to a change in estimate to workers compensation reserves. + References No definition available. + Details
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Namespace Prefix: | nwpx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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