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v2.4.1.9

Line of Credit and Long Term Debt - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Debt Instrument [Line Items] |  |  |
Borrowings outstanding under credit agreement | $ 45,600,000 us-gaap_LineOfCredit | $ 87,900,000 us-gaap_LineOfCredit |
Credit agreement maximum borrow capacity | 165,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity |  |
Additional net borrowing capacity under credit agreement | 61,700,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity |  |
Weighted average borrowing rate | 2.43%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod | 2.69%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod |
Credit agreement expiration date | Oct. 24, 2017 |  |
Remaining principal on term notes |  | 6,400,000 us-gaap_LongTermDebt |
Interest expense for continuing operations from line of credit borrowing, term notes and capital leases | 2,300,000 us-gaap_InterestExpenseDebt | 3,600,000 us-gaap_InterestExpenseDebt | 5,200,000 us-gaap_InterestExpenseDebt
Capitalized interest on line of credit borrowing, term notes and capital leases | 200,000 us-gaap_InterestCostsCapitalized | 400,000 us-gaap_InterestCostsCapitalized | 200,000 us-gaap_InterestCostsCapitalized
Term Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Remaining principal on term notes |  | $ 6,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = nwpx_TermLoanMember |
Minimum [Member] | Term Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate |  | 9.07%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = nwpx_TermLoanMember / us-gaap_RangeAxis = us-gaap_MinimumMember |
Maximum [Member] | Term Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate |  | 10.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = nwpx_TermLoanMember / us-gaap_RangeAxis = us-gaap_MaximumMember |
London Interbank Offered Rate (LIBOR) [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate | 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember | 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate | 1.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate | 2.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
Prime Rate [Member] | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate | 0.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_PrimeRateMember |  |
Prime Rate [Member] | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Credit agreement interest rate | 1.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_PrimeRateMember |  |

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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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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- Definition Amount of interest capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Allowance for Funds Used during Construction -URI http://asc.fasb.org/extlink&oid=6504829 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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- Definition Amount of the cost of borrowed funds accounted for as interest expense for debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 5 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: 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 effective interest rate during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Balance Type: | credit
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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + 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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