Acquisitions and Dispositions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands								12 Months Ended
	Jan. 17, 2018	Jul. 07, 2017	Mar. 01, 2017	Feb. 28, 2017	Aug. 29, 2016	Apr. 04, 2016	Mar. 25, 2016	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Total purchase consideration:
Cash, net of cash acquired								$ 318,474	$ 38,512	$ 471,118
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
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
ON Portfolio
Total purchase consideration:
Cash					$ 104,000
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net					4,816
Goodwill					56,084
Assets acquired and liabilities assumed					104,000
ON Portfolio | Patented and unpatented technologies
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets					8,800
ON Portfolio | Non-compete agreement
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets					2,500
ON Portfolio | Customer relationships
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets					$ 31,800
Menber's S.p.A.
Total purchase consideration:
Cash, net of cash acquired						$ 19,162
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net						12,919
Property, plant, and equipment						1,693
Goodwill						8,091
Current liabilities						(7,220)
Other non-current liabilities						(1,444)
Assets acquired and liabilities assumed						19,162
Menber's S.p.A. | Patented and unpatented technologies
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets						224
Menber's S.p.A. | Trademarks and tradenames
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets						1,849
Menber's S.p.A. | Customer relationships
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets						$ 3,050
PolySwitch
Total purchase consideration:
Original consideration							$ 350,000
Post closing consideration adjustment received							(1,708)
Acquired cash							(3,810)
Acquired cash to be returned to seller							3,810
Total purchase consideration							348,292
Allocation of consideration to assets acquired and liabilities assumed:
Current assets, net							60,228
Property, plant, and equipment							51,613
Land lease							4,290
Goodwill							165,088
Other long-term assets							11,228
Current liabilities							(35,280)
Other non-current liabilities							(5,020)
Assets acquired and liabilities assumed							348,292
PolySwitch | Patented and unpatented technologies
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets							56,425
PolySwitch | Customer relationships
Allocation of consideration to assets acquired and liabilities assumed:
Intangible assets							$ 39,720