Note 2 - Acquisition of Businesses (Details) - Unaudited Pro Forma Results (Hamlin, Inc. [Member], USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 27, 2014	Sep. 28, 2013	Sep. 27, 2014	Sep. 28, 2013
Hamlin, Inc. [Member]
Note 2 - Acquisition of Businesses (Details) - Unaudited Pro Forma Results [Line Items]
Revenues	$ 217,608	$ 201,040	$ 645,375	$ 591,095
Net income	$ 29,940	$ 26,990	$ 79,907	$ 65,425
Net income per share:
Basic	$ 1.33	$ 1.20	$ 3.55	$ 2.94
Diluted	$ 1.32	$ 1.19	$ 3.52	$ 2.91
Weighted-average shares and equivalent shares outstanding:
Basic	22,536	22,428	22,536	22,274
Diluted	22,689	22,625	22,722	22,497

Name:	lfus_NetIncomePerShareAbstract
Namespace Prefix:	lfus_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	lfus_Note2AcquisitionofBusinessesDetailsUnauditedProFormaResultsLineItems
Namespace Prefix:	lfus_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	lfus_WeightedAverageSharesAndEquivalentSharesOutstandingAbstract0
Namespace Prefix:	lfus_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The weighted average number of shares or units and dilutive common stock or unit equivalents outstanding in the calculation of proforma diluted earnings per share (earnings per unit), which is commonly presented in initial public offerings based on the terms of the offering.

+ References

No definition available.

+ Details

Name:	us-gaap_ProFormaWeightedAverageSharesOutstandingDiluted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average number of shares (units) outstanding in the calculation of pro forma basic earnings per share (earnings per unit), which is commonly presented in initial public offerings based on the terms of the offering.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageBasicSharesOutstandingProForma
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration