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Note 20 - Quarterly Data (Unaudited) (Details Textual) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Disposition of Property Plant Equipment			$ 51,000	$ (19,000)	$ (105,000)
Denver Facility [Member] | Water Transmission Group [Member]
Gain (Loss) on Disposition of Property Plant Equipment		$ 7,900,000
Cost of Sales [Member]
Workers Compensation Expense	$ 1,200,000

X

- Definition

Amount of workers compensation charge related to a change in estimate to workers compensation reserves.

+ References

No definition available.

+ Details

Name:	nwpx_WorkersCompensationExpense
Namespace Prefix:	nwpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	duration

X

- Details

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Namespace Prefix:
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Period Type:

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=nwpx_WaterTransmissionGroupMember
Namespace Prefix:
Data Type:	na
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Period Type:

X

- Details

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Namespace Prefix:
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Period Type: