XML 16 R1.htm IDEA: XBRL DOCUMENT v3.8.0.1
Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2017
Feb. 23, 2018
Jun. 30, 2017
Document Information [Line Items]      
Entity Registrant Name Northwest Pipe Co.    
Entity Central Index Key 0001001385    
Trading Symbol nwpx    
Current Fiscal Year End Date --12-31    
Entity Filer Category Accelerated Filer    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer No    
Entity Common Stock, Shares Outstanding (in shares)   9,723,883  
Entity Public Float     $ 134,068,871
Document Type 10-K    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Amendment Flag false