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

X

- Definition

The weighted average interest rate on capital leases.

+ References

No definition available.

+ Details

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Namespace Prefix:	nwpx_
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Balance Type:	na
Period Type:	instant

X

- Definition

Maximum duration of operating leases (in years).

+ References

No definition available.

+ Details

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Namespace Prefix:	nwpx_
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X

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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

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- Details

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Balance Type:	na
Period Type:	duration

X

- 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

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