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Fair value at acquisition-date of the assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interests issued by the acquirer, including but not limited to, instruments or interests issued or issuable in consideration for the business combination.

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Amount of assets that are expected to be realized or consumed within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount of liabilities due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

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Amount of liabilities assumed at the acquisition date.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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Fair value at acquisition-date of the equity interest in the acquiree held by the acquirer, immediately before the acquisition date for businesses combined in stages.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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Note 2 - Acquisitions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands					6 Months Ended
	Jan. 17, 2018	Jul. 07, 2017	Mar. 01, 2017	Feb. 28, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 30, 2017
Cash, net of cash acquired					$ 310,487	$ 14,172
Goodwill					832,227		$ 453,414
Goodwill					832,227		$ 453,414
Cash, net of cash acquired					310,487	14,172
Non-cash, fair value of commitment to purchase non-controlling interest						$ 9,000
IXYS Corporation [Member]
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
Current assets, net	155,959
Property, plant, and equipment	77,442
Intangible assets	212,720
Goodwill	382,317
Other non-current assets	31,570
Other non-current liabilities	(81,220)
	778,788
Cash	380,600
Current assets, net	155,959
Intangible assets, other than goodwill	212,720
Goodwill	382,317
	778,788
Cash, net of cash acquired	302,865
Property, plant, and equipment	$ 77,442
U.S. Sensor [Member]
Current assets, net		$ 4,635
Goodwill		16,075
		24,340
Cash		24,340
Current assets, net		4,635
Goodwill		16,075
Current liabilities		(540)
		24,340
U.S. Sensor [Member] \| Patented and Unpatented Technologies [Member]
Intangible assets		1,090
Intangible assets, other than goodwill		1,090
U.S. Sensor [Member] \| Trademarks and Trade Names [Member]
Intangible assets		200
Intangible assets, other than goodwill		200
U.S. Sensor [Member] \| Noncompete Agreements [Member]
Intangible assets		50
Intangible assets, other than goodwill		50
U.S. Sensor [Member] \| Customer Relationships [Member]
Intangible assets		2,830
Intangible assets, other than goodwill		$ 2,830
Monolith [Member]
Cash, net of cash acquired			$ 14,172	$ 14,200
Current assets, net			891
Property, plant, and equipment			789
Goodwill			20,641
Other non-current liabilities			(1,870)
Current assets, net			891
Goodwill			20,641
Current liabilities			(639)
Original investment			3,500	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
Property, plant, and equipment			789
			26,672
Monolith [Member] \| Patented and Unpatented Technologies [Member]
Intangible assets			6,720
Intangible assets, other than goodwill			6,720
Monolith [Member] \| Noncompete Agreements [Member]
Intangible assets			140
Intangible assets, other than goodwill			$ 140