XML 15 R1.htm IDEA: XBRL DOCUMENT v3.20.4
Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Feb. 22, 2021
Jun. 30, 2020
Document Information [Line Items]      
Entity Central Index Key 0001001385    
Entity Registrant Name Northwest Pipe Co.    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2020    
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 0-27140    
Entity Incorporation, State or Country Code OR    
Entity Tax Identification Number 93-0557988    
Entity Address, Address Line One 201 NE Park Plaza Drive, Suite 100    
Entity Address, City or Town Vancouver    
Entity Address, State or Province WA    
Entity Address, Postal Zip Code 98684    
City Area Code 360    
Local Phone Number 397-6250    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol NWPX    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 240,361,182
Entity Common Stock, Shares Outstanding   9,814,398