Acquisitions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands					9 Months Ended
	Jan. 17, 2018	Jul. 07, 2017	Mar. 01, 2017	Feb. 28, 2017	Sep. 29, 2018	Sep. 30, 2017	Dec. 30, 2017
Total purchase consideration:
Cash, net of cash acquired					$ 313,475	$ 38,610
Non-cash, fair value of commitment to purchase non-controlling interest					5,000	$ 9,000
Allocation of consideration to assets acquired and liabilities assumed:
Goodwill					$ 830,354		$ 453,414
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	379,619
Other non-current assets	31,570
Other non-current liabilities	(78,493)
Assets acquired and liabilities assumed	$ 778,788
U.S. Sensor
Total purchase consideration:
Cash		$ 24,340
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net		4,635
Goodwill		16,075
Current liabilities		(540)
Assets acquired and liabilities assumed		24,340
U.S. Sensor | Patented and unpatented technologies
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets		1,090
U.S. Sensor | Trademarks and tradenames
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets		200
U.S. Sensor | Non-compete agreement
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets		50
U.S. Sensor | Customer relationships
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets		$ 2,830
Monolith
Total purchase consideration:
Original investment			$ 3,500
Cash, net of cash acquired			14,172	$ 14,200
Non-cash, fair value of commitment to purchase non-controlling interest			9,000	9,000
Total purchase consideration			26,672	$ 26,700
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net			891
Property, plant, and equipment			789
Goodwill			20,641
Other non-current liabilities			(1,870)
Current liabilities			(639)
Assets acquired and liabilities assumed			26,672
Monolith | Patented and unpatented technologies
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets			6,720
Monolith | Non-compete agreement
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets			$ 140