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Fair Value Measurements - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of long-term debt |  | $ 6,300,000 us-gaap_LongTermDebtFairValue |
Carrying value of long-term debt |  | 6,400,000 us-gaap_LongTermDebt |
Non-recurring fair value measurement impairment charges |  | 300,000 nwpx_NonrecurringFairValueMeasurementImpairmentCharge | 0 nwpx_NonrecurringFairValueMeasurementImpairmentCharge
Impairment of goodwill | 16,066,000 us-gaap_GoodwillImpairmentLoss |  |
Impairment of long lived asset |  | 27,500,000 us-gaap_TangibleAssetImpairmentCharges |
Tubular Products [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Impairment of goodwill | 16,066,000 us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PublicUtilitiesInventoryTubularGoodsMember | 0 us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PublicUtilitiesInventoryTubularGoodsMember | 0 us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = us-gaap_PublicUtilitiesInventoryTubularGoodsMember
Permalok Corporation [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Contingent consideration fair value estimate of liability | $ 2,700,000 us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh / us-gaap_BusinessAcquisitionAxis = nwpx_PermalokCorporationMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | $ 4,400,000 us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh / us-gaap_BusinessAcquisitionAxis = nwpx_PermalokCorporationMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember |

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- Definition Nonrecurring fair value measurement impairment charge. + References No definition available. + Details
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Namespace Prefix: | nwpx_
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Balance Type: | debit
Period Type: | duration

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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of loss from the write-down of an asset representing the 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 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 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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Balance Type: | credit
Period Type: | instant

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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The charge against earnings resulting from the aggregate write down of tangible assets from their carrying value to their fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230 + Details
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