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Acquisitions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands		9 Months Ended
	Jan. 17, 2018	Sep. 28, 2019	Sep. 29, 2018	Dec. 29, 2018
Total purchase consideration:
Cash, net of cash acquired		$ 775	$ 313,475
Allocation of consideration to assets acquired and liabilities assumed:
Goodwill		$ 813,653		$ 826,715
IXYS Corporation
Total purchase consideration:
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