XML 35 R36.htm IDEA: XBRL DOCUMENT v2.4.1.9

Share-based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 OptionPlan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of inactive stock option plan	1nwpx_NumberOfInactiveStockOptionPlans
Number of active stock incentive plan	1nwpx_NumberOfActiveStockIncentivePlans
Minimum performance awards issued multiplier	0.00%nwpx_MinimumPerformanceAwardsIssuedMultiplier
Maximum performance awards issued multiplier	200.00%nwpx_MaximumPerformanceAwardsIssuedMultiplier
PSAs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
PSAs at target level of performance included in unvested balance	140,000nwpx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberAtTargetLevelOfPerformance / us-gaap_AwardTypeAxis = nwpx_PerformanceShareAwardsMember
Restricted Stock Units and Performance Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	3.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = nwpx_RestrictedStockUnitsAndPerformanceAwardsMember
Expected weighted average period of compensation expense to be recognized	1 year 6 months
Maximum [Member] | RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years

X

- Definition

Maximum performance awards issued multiplier.

+ References

No definition available.

+ Details

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

X

- Definition

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

X

- Definition

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

X

- Definition

Number of inactive stock option plans.

+ References

No definition available.

+ Details

Name:	nwpx_NumberOfInactiveStockOptionPlans
Namespace Prefix:	nwpx_
Data Type:	xbrli:integerItemType
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

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

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

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
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

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

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=6415400&loc=d3e5070-113901

+ Details

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

X

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

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

X

- Details

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