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Acquisitions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands		6 Months Ended
	Jan. 17, 2018	Jun. 29, 2019	Jun. 30, 2018	Dec. 29, 2018
Total purchase consideration:
Cash, net of cash acquired		$ 775	$ 310,487
Allocation of consideration to assets acquired and liabilities assumed:
Goodwill		$ 826,408		$ 826,715
IXYS Corporation
Total purchase consideration:
Cash	$ 380,600
Cash, net of cash acquired	302,865
Cash settled stock options	3,622
Littelfuse stock	434,192
Converted stock options	38,109
Total purchase consideration	778,788
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net	155,930
Property, plant, and equipment	77,442
Intangible assets	212,720
Goodwill	382,360
Other non-current assets	28,706
Other non-current liabilities	(78,370)
Assets acquired and liabilities assumed	$ 778,788