XML 67 R48.htm IDEA: XBRL DOCUMENT
v3.20.4

Summary of Significant Accounting Policies and Other Information - Narrative (Details) $ in Thousands, € in Millions |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 17, 2018 USD ($) | Sep. 26, 2020 USD ($) | Jun. 27, 2020 USD ($) | Dec. 28, 2019 USD ($) | Jun. 29, 2019 USD ($) | Mar. 30, 2019 USD ($) | Dec. 29, 2018 USD ($) | Sep. 29, 2018 USD ($) | Dec. 26, 2020 USD ($) reporting_unit | Dec. 28, 2019 USD ($) | Dec. 29, 2018 USD ($) | Dec. 26, 2020 EUR (€) | Sep. 27, 2020 | Dec. 28, 2019 EUR (€)
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of sales |  |  |  |  |  |  |  |  | $ 944,523 | $ 957,578 | $ 1,065,053 |  |  |
Selling, general, and administrative expenses |  |  |  |  |  |  |  |  | 204,507 | 220,448 | 276,329 |  |  |
Research and development expenses |  |  |  |  |  |  |  |  | 52,538 | 79,997 | 87,264 |  |  |
Equity investment |  |  |  | $ 12,969 |  |  |  |  | 19,186 | 12,969 |  |  |  |
Gain (loss) from equity method investments |  |  |  |  |  |  |  |  | 0 |  | (600) |  |  |
Equity method investments |  |  |  | 11,100 |  |  |  |  | 11,400 | 11,100 |  |  |  |
Impairment charges |  | $ 100 |  | 4,200 | $ 200 | $ 2,800 |  |  | 36,078 | 322 | 2,218 |  |  |
Goodwill impairment |  |  | $ 33,800 |  |  |  |  |  | 33,841 |  |  |  |  |
Goodwill |  |  |  | 820,589 |  |  | $ 826,715 |  | $ 816,812 | 820,589 | 826,715 |  |  |
Number of reporting units | reporting_unit |  |  |  |  |  |  |  |  | 7 |  |  |  |  |
Trade receivables, allowances |  |  |  | 1,300 |  |  |  |  | $ 1,400 | 1,300 |  |  |  |
Foreign exchange (gain) loss |  |  |  |  |  |  |  |  | (14,875) | 5,224 | (863) |  |  |
Loss contingency accrual |  |  |  | 1,100 |  |  |  |  | 2,900 | 1,100 |  | € 2.3 |  | € 1.0
TCJA one-time charge |  |  |  |  |  |  | 3,200 |  |  |  |  |  |  |
Toll charge, noncurrent |  |  |  |  |  |  |  |  | (20,800) |  |  |  |  |
Toll charge, current |  |  |  |  |  |  |  |  | 3,000 |  |  |  |  |
90 days past due |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trade receivables, allowances |  |  |  | 1,000 |  |  |  |  | 1,000 | 1,000 |  |  |  |
Other Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment charges |  |  |  |  |  |  |  |  | 100 | 7,300 |  |  |  |
Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advertising expense |  |  |  |  |  |  |  |  | 2,100 | 2,700 | 2,800 |  |  |
Shipping and handling cost and expenses | Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of sales |  |  |  |  |  |  |  |  | $ 11,100 | 11,000 | 12,300 |  |  |
Land use rights |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 50 years |  |  |  |  |
Electronics Non Silicon Unit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 218.00% |
Electronics (Silicon) Unit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 44.00% |
Passenger Car |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 160.00% |
Commercial Vehicle Products |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 137.00% |
Automotive Sensors |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill impairment |  |  | $ 33,800 |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |  |  | $ 9,800 |  |  |  |  |
Sensors |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 59.00% |
Relay Unit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 139.00% |
Fuse Unit |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  |  |  |  |  |  |  |  |  | 210.00% |
Minimum | Trademarks and tradenames |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |
Minimum | Patents, licenses and software |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |
Minimum | Distribution network |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |
Maximum | Trademarks and tradenames |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |
Maximum | Patents, licenses and software |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 17 years |  |  |  |  |
Maximum | Distribution network |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |
Building |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 21 years |  |  |  |  |
Equipment | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |
Equipment | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |
Furniture and Fixtures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 7 years |  |  |  |  |
Tools, Dies and Molds |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |
Computer Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Useful life (in years) |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |
Other Noncurrent Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  |  |  | 10,500 |  |  |  |  | $ 13,200 | 10,500 |  |  |  |
Other Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity investment |  |  |  | $ 400 |  |  |  |  | $ 500 | $ 400 |  |  |  |
Polytronics |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 7.20% |  |  |  |  | 7.20% | 7.20% |  | 7.20% |  | 7.20%
Equity investment |  |  |  | $ 12,800 |  |  |  |  | $ 19,200 | $ 12,800 |  | € 15.7 |  | € 11.6
Powersem | Equity Method |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  | 45.00% |  |  | 45.00% |  |
EB Tech | Equity Method |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  | 19.00% |  |  | 19.00% |  |
ATEC | Equity Method |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  | 24.00% |  |  | 24.00% |  |
Toll Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  |  |  |  |  |  | 2,300 |  |  |  |  |  |  |
Other Items |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  |  |  |  |  |  | 900 |  |  |  |  |  |  |
IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill | $ 382,360 |  |  |  |  |  |  |  |  |  |  |  |  |
IXYS Corporation | Toll Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge | $ 7,000 |  |  |  |  |  | $ 7,000 | $ (2,000) |  |  |  |  |  |
Revision of Prior Period |  |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Other Information |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of sales |  |  |  |  |  |  |  |  |  | (4,800) | (900) |  |  |
Selling, general, and administrative expenses |  |  |  |  |  |  |  |  |  | (7,600) | $ (11,700) |  |  |
Research and development expenses |  |  |  |  |  |  |  |  |  | $ (500) |  |  |  |

X
- Definition The noncurrent amount of income tax expense (benefit) from continuing operations attributable to an adjustment of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity referred as Toll charge. + References No definition available. + Details
Name: | lfus_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiabilityTollChargeNoncurrent
Namespace Prefix: | lfus_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Income Tax Expense Benefit Continuing Operations Adjustment of Deferred Tax Asset Liability, Toll Charge, Current + References No definition available. + Details
Name: | lfus_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentofDeferredTaxAssetLiabilityTollChargeCurrent
Namespace Prefix: | lfus_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents percentage of ownership in the long -term investment. + References No definition available. + Details
Name: | lfus_LongtermInvestmentOwnershipPercentage
Namespace Prefix: | lfus_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition [Line Items] for Summary of Significant Accounting Policies and Other Information [Table] + References No definition available. + Details
Name: | lfus_SummaryofSignificantAccountingPoliciesandOtherInformationLineItems
Namespace Prefix: | lfus_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 35 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848 + Details
Name: | us-gaap_AdvertisingExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of allowance for credit loss on accounts receivable, classified as noncurrent. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(4)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_AllowanceForDoubtfulAccountsReceivableNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228 + Details
Name: | us-gaap_AssetImpairmentCharges
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 924 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 11.L) -URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(a),(d)) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
Name: | us-gaap_CostOfGoodsAndServicesSold
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 25 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8813-108599 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_EquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121605123&loc=d3e30226-110892 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 10 -Section 45 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=120253306&loc=d3e28228-110885 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 + Details
Name: | us-gaap_ForeignCurrencyTransactionGainLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
Name: | us-gaap_Goodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13854-109267 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13777-109266 + Details
Name: | us-gaap_GoodwillImpairmentLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of income (loss) for proportionate share of equity method investee's income (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(12)) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(10)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(f)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of loss contingency liability expected to be resolved after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349 + Details
Name: | us-gaap_LossContingencyAccrualCarryingValueNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of investment in marketable security. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
Name: | us-gaap_MarketableSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number of reporting units tested for impairment of goodwill. A reporting unit is an operating segment or one level below an operating segment. + References No definition available. + Details
Name: | us-gaap_NumberOfReportingUnits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of fair value of reporting unit in excess of carrying amount. + References No definition available. + Details
Name: | us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 730 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373 + Details
Name: | us-gaap_ResearchAndDevelopmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.4) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
Name: | us-gaap_SellingGeneralAndAdministrativeExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of income tax expense (benefit) from effect of Tax Cuts and Jobs Act. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 + Details
Name: | us-gaap_TaxCutsAndJobsActOf2017IncomeTaxExpenseBenefit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=lfus_OtherExpenseIncomeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_SellingGeneralAndAdministrativeExpensesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=us-gaap_ShippingAndHandlingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_UseRightsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_ElectronicsNonSiliconUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_ElectronicsSiliconUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_PassengerCarMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_CommercialVehicleProductsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_AutomotiveSensorsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_SensorsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_RelayUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReportingUnitAxis=lfus_FuseUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksAndTradeNamesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lfus_PatentsLicensesAndSoftwareMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_DistributionRightsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_EquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ToolsDiesAndMoldsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ComputerEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=lfus_PolytronicsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=lfus_PowersemMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_EquityMethodInvesteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=lfus_EBTechMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=lfus_ATECMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TaxPeriodAxis=lfus_TollChargeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TaxPeriodAxis=lfus_OtherItemsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=lfus_IXYSCorporationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RestatementAxis=srt_RestatementAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |