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Goodwill and Intangible Assets - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 30, 2014
Goodwill [Line Items] |  |  |  |
Goodwill | $ 5,282 us-gaap_Goodwill | $ 25,760 us-gaap_Goodwill | $ 20,478 us-gaap_Goodwill |
Goodwill written off related to sale of business | 4,412 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit |  |  |
Amortization expense | 540 us-gaap_AmortizationOfIntangibleAssets | 27 us-gaap_AmortizationOfIntangibleAssets | 27 us-gaap_AmortizationOfIntangibleAssets |
Tubular Products [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 16,100 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PublicUtilitiesInventoryTubularGoodsMember |  |  |
Water Transmission [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 5,300 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_WaterTransmissionMember |  |  |
Disposal of OCTG Business [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill written off related to sale of business | $ 4,400 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember |  |  | $ 4,400 us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = nwpx_OilCountryTubularGoodsMember

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + 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=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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Period Type: | duration

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Details
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Namespace Prefix: |
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- Details
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |