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Long-Term Debt - Schedule of Long-Term Debt (Parenthetical) (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2013
Series A Term Note [Member] |
Debt Instrument [Line Items] |
Annual term note payment | $ 2,100,000
Quarterly paid interest rate | 10.50%
Maturity date | Feb. 25, 2014
Series B Term Note [Member] |
Debt Instrument [Line Items] |
Annual term note payment | 1,500,000
Quarterly paid interest rate | 10.22%
Maturity date | Jun. 21, 2014
Series C Term Note [Member] |
Debt Instrument [Line Items] |
Annual term note payment | 1,400,000
Quarterly paid interest rate | 9.11%
Maturity date | Oct. 26, 2014
Series D Term Note [Member] |
Debt Instrument [Line Items] |
Annual term note payment | $ 643,000
Quarterly paid interest rate | 9.07%
Maturity date | Jan. 24, 2015

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- Definition Annual repayment amount due on long term debt. + References No definition available. + Details
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Namespace Prefix: | nwpx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The average effective interest rate during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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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 Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 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(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration