Document And Entity Information - shares
9 Months Ended
Sep. 29, 2018
Oct. 29, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name LITTELFUSE INC /DE  
Entity Central Index Key 0000889331  
Trading Symbol lfus  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   25,156,759
Document Type 10-Q  
Document Period End Date Sep. 29, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Emerging Growth Company false  
Entity Small Business false