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Commitments and Contingencies - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended
Jun. 30, 2007
Jan. 31, 2015
Jul. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Jun. 30, 2014
Other Commitments [Line Items]            
Company made payment to Lower Willamette Group as interim settlement $ 200,000us-gaap_LitigationSettlementAmount          
Amount spent for Source Control work       100,000nwpx_AmountSpentToCompleteWorkSpecifiedInWorkPlans    
Amount spent for further Source Control work       100,000nwpx_AmountSpentForFurtherSourceControlWork    
Number of potentially responsible parties       100nwpx_NumberOfPotentiallyResponsibleParties    
Trades and other receivables       58,310,000us-gaap_ReceivablesNetCurrent 72,470,000us-gaap_ReceivablesNetCurrent  
Total stand-by letters of credit       2,300,000us-gaap_LettersOfCreditOutstandingAmount    
Subsequent Event [Member]            
Other Commitments [Line Items]            
Company made payment to Lower Willamette Group as interim settlement   2,600,000us-gaap_LitigationSettlementAmount
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		        
Settled Litigation [Member]            
Other Commitments [Line Items]            
Trades and other receivables       2,600,000us-gaap_ReceivablesNetCurrent
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember		    
Portland Harbor Natural Resources Trustee Council [Member]            
Other Commitments [Line Items]            
Injury assessment process, payment     200,000us-gaap_LossContingencyAccrualCarryingValuePayments
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= nwpx_PortlandHarborNaturalResourcesTrusteeCouncilMember		      
Injury assessment process, outstanding           400,000us-gaap_LossContingencyAccrualCarryingValueCurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= nwpx_PortlandHarborNaturalResourcesTrusteeCouncilMember
Minimum [Member]            
Other Commitments [Line Items]            
Estimated cost       169,000,000nwpx_EstimatedCostOfEpaSelectedRemedy
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Estimated years to implement       2 years    
Maximum [Member]            
Other Commitments [Line Items]            
Estimated cost       $ 1,760,000,000nwpx_EstimatedCostOfEpaSelectedRemedy
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Estimated years to implement       28 years