Document And Entity Information - USD ($)	12 Months Ended
	Dec. 29, 2018	Feb. 18, 2019	Jun. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	LITTELFUSE INC /DE
Entity Central Index Key	0000889331
Current Fiscal Year End Date	--12-29
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding		24,706,335
Entity Public Float			$ 4,409,178,694
Document Type	10-K
Document Period End Date	Dec. 29, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false