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Shareholder Rights Plan - Additional Information (Detail) (USD $)	0 Months Ended
	Jun. 18, 2009	Dec. 31, 2014 Right
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of series A junior participating preferred stock shares reserved		150,000nwpx_NumberOfSeriesJuniorParticipatingPreferredStockSharesReserved
Number of non-detachable preferred stock purchase right distributed as dividend to shareholders		1nwpx_DividendDistributionUnderShareholderRightsPlan
Right to purchase a share of Preferred Stock per each right		0.01us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
Price at which shareholders holding a right may purchase 1/100th of a share of preferred stock		$ 83.00nwpx_PurchasePriceUnderRights
Acquisition percentage to cause rights to become exercisable		15.00%nwpx_AcquisitionPercentage
Market value, multiple over exercise price		2nwpx_ValueOfExercisedRight
Value at which Company may redeem rights		$ 0.01nwpx_RightRedemptionValue
Rights agreement expiration date	Jun. 28, 2019

X

- Definition

Percentage of Company's outstanding shares of common stock which person or group must acquire or commence a tender offer to acquire to cause Rights to become exercisable.

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No definition available.

+ Details

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Period Type:	instant

X

- Definition

Amended And Restated Rights Agreement Expiration Date

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No definition available.

+ Details

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X

- Definition

Dividend distribution under shareholder rights plan.

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No definition available.

+ Details

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X

- Definition

Number of Series A Junior Participating Preferred Stock Shares reserved under the Shareholder Rights Plan.

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No definition available.

+ Details

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X

- Definition

Price at which shareholders holding a Right may purchase 1/100th of a share of preferred stock under the Shareholder Rights Plan.

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No definition available.

+ Details

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Balance Type:	na
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X

- Definition

Under certain circumstances, the value at which the Company may redeem the right.

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No definition available.

+ Details

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Namespace Prefix:	nwpx_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

X

- Definition

If exercised, the value of common stock of the Company or an acquiring company a Right holder may purchase. This amount is equal to two times the exercise price of the Right.

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No definition available.

+ Details

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X

- Definition

Number of securities into which each warrant or right may be converted. For example, but not limited to, each warrant may be converted into two shares.

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No definition available.

+ Details

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