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Segment Information (Details Textual) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 Manufacturing_Facility | Dec. 31, 2011 | Dec. 31, 2010
Segment Information (Textual) [Abstract] |  |  |
Manufacturing locations, Water Transmission | 6 |  |
Threshold for disclosure, sales represented by individual customer |  | 10.00% | 10.00%
Percent of total net sales for single customer | 12.00% |  |
MEXICO [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Property and equipment, net, located in Mexico | 2.4 | 2.4 |

X
- Definition Number of manufacturing facility locations for the Water Transmission segment. + References No definition available. + Details
Name: | nwpx_NumberOfManufacturingFacilityLocationsWaterTransmission
Namespace Prefix: | nwpx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | nwpx_PropertyPlantAndEquipmentNetLocatedInMexico
Namespace Prefix: | nwpx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | nwpx_SegmentInformationTextualAbstract
Namespace Prefix: | nwpx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Threshold for disclosure for sales represented by any one individual customer. + References No definition available. + Details
Name: | nwpx_ThresholdForDisclosureSalesRepresentedByIndividualCustomer
Namespace Prefix: | nwpx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of revenue generated from a single external customer that accounts for 10 percent or more of an entity's revenues. + References No definition available. + Details
Name: | us-gaap_EntityWideRevenueMajorCustomerPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration