Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Segment	Jun. 30, 2011
Segment Information Report
Net sales	$ 131,041	$ 143,801	$ 273,216	$ 255,259
Total Gross profit	13,577	16,671	30,077	31,439
Total Operating income	6,970	11,068	16,149	18,547
Segment Information (Textual) [Abstract]
Number of reportable segments			2
Water Transmission [Member]
Segment Information Report
Net sales	59,050	74,459	117,481	133,104
Total Gross profit	8,149	11,531	17,848	21,425
Total Operating income	6,130	9,142	14,154	17,403
Tubular Goods [Member]
Segment Information Report
Net sales	71,991	69,342	155,735	122,155
Total Gross profit	5,428	5,140	12,229	10,014
Total Operating income	4,651	4,304	10,847	8,153
Corporate [Member]
Segment Information Report
Total Operating income	$ (3,811)	$ (2,378)	$ (8,852)	$ (7,009)

Name:	nwpx_SegmentInformationTextualAbstract
Namespace Prefix:	nwpx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

+ References

+ Details

Name:	us-gaap_GrossProfit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

Name:	us-gaap_SalesRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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