Name:	nwpx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedPeriodForRecognitionInYears
Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Maximum performance awards issued multiplier.

No definition available.

Name:	nwpx_MaximumPerformanceAwardsIssuedMultiplier
Namespace Prefix:	nwpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Minimum performance awards issued multiplier.

No definition available.

Name:	nwpx_MinimumPerformanceAwardsIssuedMultiplier
Namespace Prefix:	nwpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	nwpx_ShareBasedCompensationAdditionalTextualAbstract
Namespace Prefix:	nwpx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The number of non-vested equity-based payment instruments at a target level of performance, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

No definition available.

Name:	nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberAtTargetLevelOfPerformance
Namespace Prefix:	nwpx_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Period over which an employee's right to exercise a PSA is no longer contingent on satisfaction of a service condition, market condition, or a performance condition, in years.

No definition available.

Name:	nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceAwardsVestingPeriod
Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period over which an employee's right to exercise a RSU is no longer contingent on satisfaction, in years.

No definition available.

Name:	nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardRestrictedStockUnitsVestingPeriod
Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Share based compensation arrangement by stock based payment award grant date fair value.

No definition available.

Name:	nwpx_ShareBasedCompensationArrangementByStockBasedPaymentAwardGrantDateFairValue
Namespace Prefix:	nwpx_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	nwpx_ShareBasedCompensationTextualAbstract
Namespace Prefix:	nwpx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Aggregate unrecognized cost of share-based awards, other than options, made to employees under an equity-based compensation plan, that have yet to vest.

No definition available.

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

The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The number of shares or units of entity securities issued in the transaction in which equity securities were issued to pay for goods or nonemployee services.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 50

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 65

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(iv)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901