Cover Page - shares	9 Months Ended
	Sep. 26, 2020	Oct. 23, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 26, 2020
Document Transition Report	false
Entity File Number	0-20388
Entity Registrant Name	LITTELFUSE INC /DE
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-3795742
Entity Address, Address Line One	8755 West 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
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	LFUS
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,375,049
Entity Central Index Key	0000889331
Current Fiscal Year End Date	--12-26
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false