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Earnings Per Share - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 17, 2018	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded (in shares)		222,526	129,658	42,305
Purchase of common stock (in shares)		175,110	579,916	391,972
Repurchases of common stock		$ 22,927	$ 95,036	$ 67,908
IXYS Corporation
Antidilutive Securities Excluded from Computation of Earnings Per Share
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
Conversion of stock (in shares)	500,000