Document and Entity Information - shares
6 Months Ended
Jun. 28, 2019
Aug. 01, 2019
Document and Entity Information    
Document Type 10-Q  
Document Period End Date Jun. 28, 2019  
Entity Registrant Name Willdan Group, Inc.  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   11,244,856
Entity Central Index Key 0001370450  
Current Fiscal Year End Date --12-28  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false