| Document and Entity Information | Jan. 29, 2020 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Jan. 29, 2020 |
| Entity Registrant Name | LITTELFUSE INC /DE |
| Entity Central Index Key | 0000889331 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 0-20388 |
| Entity Tax Identification Number | 36-3795742 |
| Entity Address, Address Line One | 8755 W. Higgins Road |
| Entity Address, Address Line Two | Suite 500 |
| Entity Address, City or Town | Chicago |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60631 |
| City Area Code | 773 |
| Local Phone Number | 628-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | LFUS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
