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Note 14 - Shareholder Rights Plan (Details Textual) - $ / shares | Jun. 18, 2009 | Jun. 17, 2009 | Dec. 31, 2018
--- | --- | --- | ---
Number of Series Junior Participating Preferred Stock Shares Reserved |  |  | 150,000
Dividend Distribution Under Shareholder Rights Plan |  |  | 1
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right |  |  | 0.01
Purchase Price Under Rights |  |  | $ 83
Acquisition Percentage |  |  | 15.00%
Value of Exercised Right |  |  | 2
Right Redemption Value |  |  | $ 0.01
Amended and Restated Rights Agreement Expiration Date | Jun. 28, 2019 | Jun. 28, 2009 |

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. + References No definition available. + Details
Name: | nwpx_AcquisitionPercentage
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the amended and restated rights agreement expiration date + References No definition available. + Details
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Namespace Prefix: | nwpx_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of non-detachable preferred stock purchase rights per share of common stock. + References 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. + References No definition available. + Details
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Namespace Prefix: | nwpx_
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Balance Type: | na
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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. + References 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. + References No definition available. + Details
Name: | nwpx_RightRedemptionValue
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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. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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. + References No definition available. + Details
Name: | us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
Namespace Prefix: | us-gaap_
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