Note 8 - Share-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015
Restricted Stock Units and Performance Stock Awards [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 1.9
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 109 days
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number At Target Level Of Performance	47,000
Restricted Stock Units (RSUs) [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Director [Member]
Share-based Goods and Nonemployee Services Transaction, Quantity of Securities Issued	22,964	10,464
Share Based Compensation Arrangement by Stock Based Payment Award Grant Date Fair Value	$ 9.58	$ 21.02
Number Of Active Stock Incentive Plans	1
Number Of Inactive Stock Option Plans	1
Minimum Performance Awards Issued Multiplier	0.00%
Maximum Performance Awards Issued Multiplier	200.00%

The maximum amount of performance awards issued multiplier.

No definition available.

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

- Definition

The number of minimum performance awards issued multiplier.

+ References

No definition available.

+ Details

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

- Definition

The number of active stock incentive plans.

+ References

No definition available.

+ Details

Name:	nwpx_NumberOfActiveStockIncentivePlans
Namespace Prefix:	nwpx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of inactive stock options plans.

+ References

No definition available.

+ Details

Name:	nwpx_NumberOfInactiveStockOptionPlans
Namespace Prefix:	nwpx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

No definition available.

+ Details

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

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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=68057994&loc=d3e25284-112666

Reference 2: 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

+ Details

Name:	us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=nwpx_RestrictedStockUnitsAndPerformanceStockAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_DirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: