Cover Page - USD ($)	12 Months Ended
	Dec. 26, 2020	Feb. 12, 2021	Jun. 27, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 26, 2020
Current Fiscal Year End Date	--12-26
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, Street	8755 West Higgins Road
Entity Address, Suite	Suite 500
Entity Address, City	Chicago
Entity Address, State	IL
Entity Address, Postal Zip Code	60631
City Area Code	773
Local Phone Number	628-1000
Title of Each Class	Common Stock, $0.01 par value
Trading Symbol	LFUS
Name of Each Exchange On Which Registered	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	true
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 3,956,326,119
Entity Common Stock, Shares Outstanding		24,537,415
Documents Incorporated by Reference	Portions of the Littelfuse, Inc. Proxy Statement for the 2021 Annual Meeting of Stockholders (the “Proxy Statement”) are incorporated by reference into Part III of this Form 10-K.
Entity Central Index Key	0000889331
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false