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Earnings Per Share - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 17, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded (in shares)		129,658	42,305	37,443
Purchase of common stock (in shares)		579,916	391,972
Repurchases of common stock		$ 95,036	$ 67,908
IXYS Corporation
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issues for acquisitions (in shares)	2,100,000
IXYS Corporation \| Conversion of IXYS Equity Awards Into Littlefuse Equity Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Conversion of stock (in shares)	500,000