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Earnings Per Share - Narrative (Details) - shares		12 Months Ended
Earnings Per Share - Narrative (Details) - shares	Jan. 17, 2018	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded (in shares)		42,305	37,443	53,448
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