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Note 5 - Share-based Compensation (Details Textual) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
Number Of Active Stock Incentive Plans	1
Number Of Inactive Stock Option Plans	1
Minimum Performance Awards Issued Multiplier	0.00%
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	$ 0.7
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	292 days
Restricted Stock Units (RSUs) [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

X

- Definition

The number of minimum performance awards issued multiplier.

+ References

No definition available.

+ Details

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

X

- Definition

The number of active stock incentive plans.

+ References

No definition available.

+ Details

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

X

- Definition

The number of inactive stock options plans.

+ References

No definition available.

+ Details

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

X

- 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=96867065&loc=d3e5070-113901

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Period Type:	duration

X

- Definition

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

+ 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=96867065&loc=d3e5070-113901

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- 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=96867065&loc=d3e5070-113901

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X

- Details

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Namespace Prefix:
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X

- Details

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X

- Details

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Namespace Prefix:
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Period Type: