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Leases - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Leases [Abstract] |  |  |
Capital lease obligations outstanding | $ 2,400,000 us-gaap_CapitalLeaseObligations |  |
Weighted average interest rate on capital leases | 9.66%nwpx_CapitalLeaseWeightedAverageInterestRate |  |
Interest expense on discontinued operations related to capital lease | 99,000 us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense | 344,000 us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense | 411,000 us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense
Rental expense from continuing operations | $ 3,200,000 us-gaap_OperatingLeasesRentExpenseNet | $ 2,800,000 us-gaap_OperatingLeasesRentExpenseNet | $ 2,900,000 us-gaap_OperatingLeasesRentExpenseNet
Maximum duration of operating leases (in years) | 10 years |  |

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- Definition The weighted average interest rate on capital leases. + References No definition available. + Details
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- Definition Maximum duration of operating leases (in years). + References No definition available. + Details
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- Definition Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date. + 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 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition Amount of interest expense allocated to disposal group, including a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section S99 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=26872618&loc=d3e7436-122677 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1063-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1060-107759 + Details
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- Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717 + Details
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