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Share-Based Compensation Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 OptionPlan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of inactive stock option plan	1
Number of active stock incentive plan	1
RSUs and PSAs at target level of performance included in unvested balance	129,000
Minimum performance awards issued multiplier	0.00%
Maximum performance awards issued multiplier	200.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of market based PSAs	2 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of market based PSAs	3 years
Restricted Stock Units and Performance Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 3.1
Expected weighted average period of compensation expense to be recognized	1 year 7 months 6 days

X

- Definition

Maximum performance awards issued multiplier.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

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

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

Number of inactive stock option plans.

+ References

No definition available.

+ Details

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

X

- 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

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Performance Stock Award Vesting Period

+ References

No definition available.

+ Details

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Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
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=6415400&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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X

- Details

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