Littelfuse (LFUS) 10-Q 31 Oct 18 2018 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 29, 2018	Oct. 29, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	LITTELFUSE INC /DE
Entity Central Index Key	889,331
Trading Symbol	lfus
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		25,156,759
Document Type	10-Q
Document Period End Date	Sep. 29, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Emerging Growth Company	false
Entity Small Business	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Current assets:
Cash and cash equivalents	$ 498,215	$ 429,676
Short-term investments	35	35
Trade receivables, less allowances (September 29, 2018 - $36,392; December 30, 2017 - $27,516)	251,644	182,699
Inventories	247,255	140,789
Prepaid income taxes and income taxes receivable	6,802	1,689
Prepaid expenses and other current assets	48,683	37,452
Total current assets	1,052,634	792,340
Property, plant, and equipment:
Land	29,528	9,547
Buildings	119,380	86,599
Equipment	569,550	505,838
Accumulated depreciation and amortization	(374,575)	(351,407)
Net property, plant, and equipment	343,883	250,577
Intangible assets, net of amortization	377,151	203,850
Goodwill	830,354	453,414
Investments	29,084	10,993
Deferred income taxes	8,979	11,858
Other assets	21,401	17,070
Total assets	2,663,486	1,740,102
Current liabilities:
Accounts payable	129,871	101,844
Accrued payroll	52,208	49,962
Accrued expenses	74,084	48,994
Accrued severance	901	1,459
Accrued income taxes	36,746	16,285
Current portion of long-term debt	10,076	6,250
Total current liabilities	303,886	224,794
Long-term debt, less current portion	690,637	489,361
Deferred income taxes	49,262	17,069
Accrued post-retirement benefits	32,901	18,742
Other long-term liabilities	67,404	62,580
Shareholders’ equity:
Common stock, par value $0.01 per share: 34,000,000 shares authorized; shares issued, September 29, 2018–25,630,347; December 30, 2017–22,713,198	254	229
Treasury stock, at cost: 475,994 and 439,598 shares, respectively	(48,546)	(41,294)
Additional paid-in capital	830,612	310,012
Accumulated other comprehensive loss	(97,631)	(63,668)
Retained earnings	834,577	722,140
Littelfuse, Inc. shareholders’ equity	1,519,266	927,419
Non-controlling interest	130	137
Total equity	1,519,396	927,556
Total liabilities and equity	$ 2,663,486	$ 1,740,102

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Statement of Financial Position [Abstract]
Allowance for trade receivables	$ 36,392	$ 27,516
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock authorized (in shares)	34,000,000	34,000,000
Common stock issued (in shares)	25,630,347	22,713,198
Treasury stock (in shares)	475,994	439,598

Condensed Consolidated Statemen

Condensed Consolidated Statements of Net Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales	$ 439,191	$ 317,889	$ 1,316,187	$ 916,685
Cost of sales	259,597	184,238	817,983	536,776
Gross profit	179,594	133,651	498,204	379,909
Selling, general, and administrative expenses	69,782	56,759	220,540	156,899
Research and development expenses	20,454	11,991	65,742	36,872
Amortization of intangibles	13,130	6,292	38,501	18,407
Total operating expenses	103,366	75,042	324,783	212,178
Operating income	76,228	58,609	173,421	167,731
Interest expense	5,775	3,467	16,980	9,868
Foreign exchange loss (gain)	982	632	(6,372)	(1,483)
Other expense (income), net	1,259	(1,013)	(2,362)	(962)
Income before income taxes	68,212	55,523	165,175	160,308
Income taxes	14,666	12,715	33,275	29,970
Net income	$ 53,546	$ 42,808	$ 131,900	$ 130,338
Income per share:
Basic (in dollars per share)	$ 2.13	$ 1.88	$ 5.31	$ 5.75
Diluted (in dollars per share)	$ 2.10	$ 1.87	$ 5.23	$ 5.69
Weighted-average shares and equivalent shares outstanding:
Basic (in shares)	25,109	22,713	24,817	22,678
Diluted (in shares)	25,471	22,953	25,212	22,906

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 53,546	$ 42,808	$ 131,900	$ 130,338
Other comprehensive (loss) income:
Pension and postemployment adjustment, net of tax	(115)	(60)	648	(441)
Unrealized loss on investments	0	(1,710)	0	(1,235)
Foreign currency translation adjustments	(7,832)	1,531	(24,816)	2,912
Comprehensive income	$ 45,599	$ 42,569	$ 107,732	$ 131,574

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017
Operating activities
Net income	$ 131,900	$ 130,338
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	37,559	28,228
Amortization of intangibles	38,501	18,407
Provision for bad debts	83	1,586
Deferred revenue	3,965	0
Non-cash inventory charges	36,927	1,607
Impairment charges	1,125	0
Loss on sale of property, plant, and equipment	511	584
Stock-based compensation	23,153	12,437
Unrealized gain on investments	(350)	0
Deferred income taxes	(10,979)	1,863
Changes in operating assets and liabilities:
Trade receivables	(20,588)	(26,792)
Inventories	(17,624)	(17,159)
Accounts payable	17,033	9,448
Accrued expenses (including post-retirement)	11,523	1,757
Accrued payroll and severance	(5,330)	(3,788)
Accrued income taxes	14,543	7,267
Prepaid expenses and other assets	(9,836)	15,537
Net cash provided by operating activities	252,116	181,320
Investing activities
Acquisitions of businesses, net of cash acquired	(313,475)	(38,610)
Proceeds from maturities of short-term investments	0	3,739
Decrease in entrusted loan	0	3,599
Purchases of property, plant, and equipment	(55,946)	(48,470)
Proceeds from sale of property, plant, and equipment	858	541
Net cash used in investing activities	(368,563)	(79,201)
Financing activities
Proceeds of revolving credit facility	60,000	15,000
Proceeds of term loan	75,000	0
Net proceeds from senior notes payable	175,000	125,000
Payments of term loan	(42,525)	(4,687)
Payments of revolving credit facility	(60,000)	(112,500)
Net proceeds (payments) related to stock-based award activities	17,920	(2,336)
Payments of entrusted loan	0	(3,599)
Debt issuance costs	(878)	(2)
Cash dividends paid	(29,258)	(23,367)
Net cash provided by (used in) financing activities	195,259	(6,491)
Effect of exchange rate changes on cash and cash equivalents	(10,273)	2,076
Increase in cash and cash equivalents	68,539	97,704
Cash and cash equivalents at beginning of period	429,676	275,124
Cash and cash equivalents at end of period	498,215	372,828
Supplemental disclosure of non-cash investing activities:
Fair value of commitment to purchase non-controlling interest of Monolith	$ 5,000	$ 9,000

Summary of Significant Accounti

Summary of Significant Accounting Policies and Other Information	9 Months Ended
	Sep. 29, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies and Other Information	Summary of Significant Accounting Policies and Other Information Nature of Operations Littelfuse, Inc. and subsidiaries (the “Company”) is a global manufacturer of leading technologies in circuit protection, power control and sensing. The Company has products in automotive and commercial vehicles, industrial applications, data and telecommunications, medical devices, consumer electronics and appliances. With a diverse and extensive product portfolio of fuses, semiconductors, polymers, ceramics, relays and sensors, the Company works with its customers to build safer, more reliable and more efficient products for the connected world in virtually every market that uses electrical energy. The Company has a network of global engineering centers and labs that develop new products and product enhancements, provides customer application support and test products for safety, reliability, and regulatory compliance. Basis of Presentation The Company’s accompanying unaudited Condensed Consolidated Financial Statements have been prepared in accordance with U.S. Generally Accepted Accounting Principles (“GAAP”) for interim financial information, the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, certain information and disclosures normally included in the consolidated balance sheets, statements of net income and comprehensive income and cash flows prepared in conformity with U.S. GAAP have been condensed or omitted as permitted by such rules and regulations, although the Company believes that the disclosures made are adequate to make the information not misleading. They have been prepared in accordance with accounting policies described in the Company’s Annual Report on Form 10-K for the year ended December 30, 2017 , which should be read in conjunction with the disclosures therein. In the opinion of management, all adjustments considered necessary for a fair presentation have been included and are of a normal, recurring nature. Operating results for interim periods are not necessarily indicative of annual operating results. Revenue Recognition Adoption On December 31, 2017, the Company adopted new guidance on revenue from contracts with customers using the modified retrospective method. The adoption did not have a significant impact on the Company’s consolidated financial statements. Revenue Disaggregation The following table disaggregates the Company’s revenue by primary business units for the three and nine months ended September 29, 2018 : Three Months Ended September 29, 2018 (in thousands) Electronics Segment Automotive Segment Industrial Segment Total Electronics – Passive Products and Sensors $ 124,174 $ — $ — $ 124,174 Electronics – Semiconductor 172,298 — — 172,298 Passenger Car Products — 57,761 — 57,761 Automotive Sensors — 27,311 — 27,311 Commercial Vehicle Products — 29,344 — 29,344 Industrial Products — — 28,303 28,303 Total $ 296,472 $ 114,416 $ 28,303 $ 439,191 Nine Months Ended September 29, 2018 (in thousands) Electronics Segment Automotive Segment Industrial Segment Total Electronics – Passive Products and Sensors $ 366,990 $ — $ — $ 366,990 Electronics – Semiconductor 493,250 — — 493,250 Passenger Car Products — 184,922 — 184,922 Automotive Sensors — 89,362 — 89,362 Commercial Vehicle Products — 93,434 — 93,434 Industrial Products — — 88,229 88,229 Total $ 860,240 $ 367,718 $ 88,229 $ 1,316,187 See Note 13, Segment Information for net sales by segment and countries. Revenue Recognition The Company recognizes revenue on product sales in the period in which the Company satisfies its performance obligation and control of the product is transferred to the customer. The Company’s sales arrangements with customers are predominately short term in nature and generally provide for transfer of control at the time of shipment as this is the point at which title and risk of loss of the product transfers to the customer. At the end of each period, for those shipments where title to the products and the risk of loss and rewards of ownership do not transfer until the product has been received by the customer, the Company adjusts revenues and cost of sales for the delay between the time that the products are shipped and when they are received by the customer. The amount of revenue recorded reflects the consideration to which the Company expects to be entitled in exchange for goods and may include adjustments for customer allowance, rebates and price adjustments. The Company’s distribution channels are primarily through direct sales and independent third-party distributors. The Company has elected the practical expedient under Accounting Standards Codification ("ASC") 340-40-25-4 to expense commissions when incurred as the amortization period of the commission asset the Company would have otherwise recognized is less than one year. Revenue and Billing The Company generally accepts orders from customers through receipt of purchase orders or electronic data interchange based on written sales agreements and purchasing contracts. Contract pricing and selling agreement terms are based on market factors, costs, and competition. Pricing is often negotiated as an adjustment (premium or discount) from the Company’s published price lists. The customer is invoiced when the Company’s products are shipped to them in accordance with the terms of the sales agreement. As the Company’s standard payment terms are less than one year, the Company has elected the practical expedient under ASC 606-10-32-18 to not assess whether a contract has a significant financing component. The Company also elected the practical expedient provided in ASC 606-10-25-18B to treat all product shipping and handling activities as fulfillment activities, and therefore recognize the gross revenue associated with the contract, inclusive of any shipping and handling revenue. This is similar to the Company’s prior practice and therefore the effect of the new guidance is immaterial. Ship and Debit Program Some of the terms of the Company’s sales agreements and normal business conditions provide customers (distributors) the ability to receive price adjustments on products previously shipped and invoiced. This practice is common in the industry and is referred to as a “ship and debit” program. This program allows the distributor to debit the Company for the difference between the distributors’ contracted price and a lower price for specific transactions. Under certain circumstances (usually in a competitive situation or large volume opportunity), a distributor will request authorization for pricing allowances to reduce its price. When the Company approves such a reduction, the distributor is authorized to “debit” its account for the difference between the contracted price and the lower approved price. The Company establishes reserves for this program based on historic activity and actual authorizations for the debit and recognizes these debits as a reduction of revenue. Return to Stock The Company has a return to stock policy whereby certain customers, with prior authorization from Littelfuse management, can return previously purchased goods for full or partial credit. The Company establishes an estimated allowance for these returns based on historic activity. Sales revenue and cost of sales are reduced to anticipate estimated returns. Volume Rebates The Company offers volume based sales incentives to certain customers to encourage greater product sales. If customers achieve their specific quarterly or annual sales targets, they are entitled to rebates. The Company estimates the projected amount of rebates that will be achieved by the customer and recognizes this estimated cost as a reduction to revenue as products are sold. Recently Adopted Accounting Standards In March 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2017-07 “Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post-Retirement Benefit Cost,” which changed the presentation of net periodic pension and post-retirement benefit cost (net benefit cost) within the Statement of Income. Under the previous guidance, net benefit cost was reported as an employee cost within operating income. The amendment required the bifurcation of net benefit cost, with the service cost component to be presented with other employee compensation costs in operating income while the other components will be reported separately outside of income from operations. ASU No. 2017-07 was effective for the first quarter of 2018 with the Company adopting the new standard on December 31, 2017. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Recognition and Measurement of Financial Assets and Financial Liabilities” which addressed certain aspects of the recognition, measurement, presentation and disclosure of financial instruments. The ASU requires the Company to recognize any changes in the fair value of certain equity investments in net income. Previously these changes were recognized in other comprehensive income ("OCI"). The Company adopted the new standard on December 31, 2017, on a modified retrospective basis, recognizing the cumulative effect as a $9.8 million increase to retained earnings. As a result of the adoption of the new standard and change in fair value of our equity investments, for the nine months ended September 29, 2018 , the Company recognized an unrealized loss of $0.3 million in Other (income) expense, net in the Condensed Consolidated Statements of Net Income. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers” (Topic 606) which supersedes the revenue recognition requirements in ASC 605, “Revenue Recognition.” This ASU provides a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most current revenue recognition guidance. The guidance permits two implementation approaches, one requiring retrospective application of the new standard with restatement of prior years and one requiring prospective application of the new standard with disclosure of results under old standards. The Company adopted the new standard on December 31, 2017 using the modified retrospective method, however, no adjustment to retained earnings was needed. The new guidance did not have a material effect on the Company’s Condensed Consolidated Statements of Net Income. See the Revenue Recognition section above for further discussion. In October 2016, the FASB issued ASU No. 2016-16, "Income Taxes” (Topic 740). This ASU update requires entities to recognize the income tax consequences of many intercompany asset transfers at the transaction date. The seller and buyer will immediately recognize the current and deferred income tax consequences of an intercompany transfer of an asset other than inventory. The tax consequences were previously deferred. The Company adopted the new standard on December 31, 2017 and it did not have a material impact. Recently Issued Accounting Standards In February 2016, the FASB issued ASU No. 2016-02, "Leases" (Topic 842). This ASU requires lessees to recognize, on the balance sheet, assets and liabilities for the rights and obligations created by leases of greater than twelve months. The accounting by lessors will remain largely unchanged. The ASU is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted. Adoption will require a modified retrospective transition. The Company will adopt the standard in the first quarter of 2019. The Company has made progress on assessing the Company’s portfolio of leases and compiling a central repository of all active leases. We are in the process of assessing the design of the future lease process and drafting a policy to address the new standard requirements. Key lease data elements are being evaluated including developing a methodology for determining the incremental borrowing rate across all countries where we have operations. While the Company has not yet completed its evaluation of the impact the new lease accounting standard will have on its Consolidated Financial Statements, the Company expects to recognize right of use assets and lease liabilities for its operating leases in the Consolidated Balance Sheet upon adoption. In January 2018, the FASB released guidance on the accounting for tax on the global intangible low-taxed income ("GILTI") provisions of the 2017 U.S. Tax Cuts and Jobs Act (the “Tax Act”). The GILTI provisions impose a tax on foreign income in excess of a deemed return on tangible assets of foreign corporations. The guidance indicates that either accounting for deferred taxes related to GILTI inclusions or treating any taxes on GILTI inclusions as period cost are both acceptable methods subject to an accounting policy election. The Company has not yet completed its assessment and therefore has not yet elected an accounting policy. In February 2018, the FASB issued ASU No. 2018-02 “Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income,” which permits the reclassification of tax effects stranded in accumulated other comprehensive income to retained earnings as a result of the Tax Act. The standard also requires entities to disclose whether or not they elected to reclassify the tax effects related to the Tax Act as well as their policy for releasing income tax effects from accumulated other comprehensive income. The standard allows the option of applying either a retrospective adoption, meaning the standard is applied to all periods in which the effect of the Tax Act is recognized, or applying the amendments in the period of adoption, meaning an adjustment is made to shareholder’s equity as of the beginning of the reporting period. ASU 2018-02 will be effective in the first quarter of 2019; however early adoption is permitted for interim and annual periods, including the reporting period in which the Tax Act was enacted. The Company is currently evaluating the impact of ASU 2018-02 on the Consolidated Financial Statements. In August 2018, the FASB issued ASU No. 2018-13, "Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement." ASU 2018-13 modified the disclosure requirements in Topic 820, "Fair Value Measurement," based on the FASB Concepts Statement, "Conceptual Framework for Financial Reporting - Chapter 8: Notes to Financial Statements," including consideration of costs and benefits. The guidance is effective for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years, with early adoption permitted. The company is currently evaluating the potential effects of this guidance on its Consolidated Financial Statements. In August 2018, the FASB issued ASU No. 2018-14 "Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans, which amends ASC 715-20, Compensation - Retirement Benefits - Defined Benefit Plans - General. The amended guidance modifies the disclosure requirements for employers that sponsor defined benefit pension or other post-retirement plans by removing and adding certain disclosures for these plans. The eliminated disclosures include (a) the amounts in OCI expected to be recognized in net periodic benefit costs over the next fiscal year, and (b) the effects of a one percentage point change in assumed health care cost trend rates on the net periodic benefit costs and the benefit obligation for post-retirement health care benefits. Additional disclosures include descriptions of significant gains and losses affecting the benefit obligation for the period. The adoption of this guidance will modify our disclosures but will not have a material effect on our Consolidated Financial Statements. In August 2018, the FASB issued ASU No. 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (a consensus of the FASB Emerging Issues Task Force).” ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The guidance is effective for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years, with early adoption permitted. The Company is currently evaluating this guidance on its Consolidated Financial Statements.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 29, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions The Company accounts for acquisitions using the acquisition method in accordance with ASC 805, “Business Combinations,” in which assets acquired and liabilities assumed are recorded at fair value as of the date of acquisition. The operating results of the acquired business are included in the Company’s Consolidated Financial Statements from the date of the acquisition. IXYS Corporation On January 17, 2018, the Company acquired IXYS Corporation (“IXYS”), a global pioneer in the power semiconductor and integrated circuit markets with a focus on medium to high voltage power control semiconductors across the industrial, communications, consumer and medical markets. IXYS has a broad customer base, serving more than 3,500 customers through its direct sales force and global distribution partners. The acquisition of IXYS is expected to accelerate the Company’s growth across the power control market driven by IXYS’s extensive power semiconductor portfolio and technology expertise. With IXYS, the Company will be able to diversify and expand its presence within industrial electronics markets, leveraging the strong IXYS industrial OEM customer base. The Company also expects to increase long-term penetration of its power semiconductor portfolio in automotive markets, expanding its global content per vehicle. Upon completion of the acquisition, at IXYS stockholders’ election and subject to proration, each share of IXYS common stock, par value $0.01 per share, owned immediately prior to the effective time was canceled and extinguished and automatically converted into the right to receive: (i) $23.00 in cash (subject to applicable withholding tax), without interest (referred to as the cash consideration), or (ii) 0.1265 of a share of common stock, par value $0.01 per share, of Littelfuse (referred to as the stock consideration). IXYS stockholders received cash in lieu of any fractional shares of Littelfuse common stock that the IXYS stockholders would otherwise have been entitled to receive. Additionally, each outstanding option to purchase shares of IXYS common stock granted under an IXYS equity plan were assumed by Littelfuse and converted into an option to acquire (i) a number of shares of Littelfuse common stock equal to the number of shares of IXYS common stock subject to such option immediately prior to the effective time multiplied by 0.1265 , rounded down to the nearest whole share, with (ii) an exercise price per share of Littelfuse common stock equal to the exercise price of such IXYS stock option immediately prior to the effective time divided by 0.1265 , rounded up to the nearest whole cent. Based on the $207.5 per share opening price of Littelfuse common stock on January 17, 2018, the consideration IXYS stockholders received in exchange of their IXYS common stock in the acquisition had a value of $814.8 million comprised of $380.6 million of cash and $434.2 million of Littelfuse stock. In addition to the consideration transferred related to IXYS common stock, the value of consideration transferred, and included in the purchase price, related to IXYS stock options that were converted to Littelfuse stock options, or cash settled, had a value of $41.7 million . As a result, total consideration was valued at $856.5 million . The total purchase price of $856.5 million has been allocated, on a preliminary basis, to assets acquired and liabilities assumed, as of the completion of the acquisition, based on preliminary estimated fair values. The purchase price allocation is preliminary because the evaluations necessary to assess the fair values of the net assets acquired are still in process. The primary area that is not yet finalized relates to the completion of the computation of the adoption of the Tax Act. As a result, these allocations are subject to change during the purchase price allocation period as the computation is finalized. The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the IXYS acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: 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 ) $ 778,788 Included in IXYS’s current assets, net was approximately $49.1 million of receivables. All IXYS goodwill, other assets and liabilities were recorded in the Electronics segment and primarily reflected in the Americas and European geographic areas. The goodwill resulting from this acquisition consists largely of the Company’s expected future product sales and synergies from combining IXYS’s products and technology with the Company’s existing electronics product portfolio. Goodwill resulting from the IXYS acquisition is not expected to be deductible for tax purposes. Included in the Company’s Condensed Consolidated Statements of Net Income for the three and nine months ended September 29, 2018 are net sales of approximately $99.7 million and $286.2 million , respectively, and a income (loss) before income taxes of $6.2 million and $(25.5) million , respectively, since the January 17, 2018 acquisition of IXYS. The Company recognized approximately $4.3 million and $11.8 million of stock compensation expense related to IXYS stock options converted to Littelfuse stock options during the three and nine months ended September 29, 2018 , of which $4.5 million was recognized immediately as it related to prior service periods. As required by purchase accounting rules, the Company recorded a $36.9 million step-up of inventory to its fair value as of the acquisition date based on the preliminary valuation. The step-up was fully amortized as a non-cash charge to cost of goods sold during the first and second quarters of 2018, as the acquired inventory was sold, and reflected as other non-segment costs. During the nine months ended September 29, 2018 , the Company incurred approximately $11.0 million of legal and professional fees related to this acquisition which were primarily recognized as selling, general, and administrative expenses . These costs were reflected as other non-segment costs. 2017 Acquisitions U.S. Sensor On July 7, 2017, the Company acquired the assets of U.S. Sensor Corporation (“U.S. Sensor”). The acquisition purchase price of $24.3 million , net of the finalization of an income tax gross up which was settled in the fourth quarter of 2017, was funded with available cash. The acquired business expands the Company’s existing sensor portfolio in several key electronics and industrial end markets. U.S. Sensor manufactures a variety of high quality negative temperature coefficient thermistors as well as thermistor probes and assemblies. Product lines also include thin film platinum resistance temperature detectors (“RTDs”) and RTD assemblies. The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the U.S. Sensor acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: Cash $ 24,340 Allocation of consideration to assets acquired and liabilities assumed: Current assets, net $ 4,635 Patented and unpatented technologies 1,090 Trademarks and tradenames 200 Non-compete agreement 50 Customer relationships 2,830 Goodwill 16,075 Current liabilities (540 ) $ 24,340 Included in U.S. Sensor’s current assets, net was approximately $1.5 million of receivables. All U.S. Sensor goodwill, other assets and liabilities were recorded in the Electronics segment and reflected in the United States geographic area. The goodwill resulting from this acquisition consists largely of the Company’s expected future product sales and synergies from combining U.S. Sensor’s products and technology with the Company’s existing electronics product portfolio. Goodwill for the above acquisition is expected to be deductible for tax purposes. As required by purchase accounting rules, the Company recorded a $1.6 million step-up of inventory to its fair value as of the acquisition date based on the preliminary valuation. The step-up was amortized as a non-cash charge to cost of goods sold during the third quarter of 2017, as the acquired inventory was sold, and reflected as other non-segment costs. Monolith In December 2015, the Company invested $3.5 million in the preferred stock of Monolith Semiconductor Inc. (“Monolith”), a U.S. start-up Company developing silicon carbide technology, which represented approximately 12% of the common stock of Monolith on an as-converted basis. The Company accounted for its investment in Monolith under the cost method with any changes in value recorded in other comprehensive income. The value of the Monolith investment was $3.5 million at December 31, 2016. On February 28, 2017, pursuant to a Securities Purchase Agreement between the Company and the stockholders of Monolith (“Securities Purchase Agreement”) and conditioned on Monolith achieving a product development milestone and other provisions, the Company acquired 62% of the outstanding common stock of Monolith for $15.0 million . The Securities Purchase Agreement includes provisions whereby the Company will acquire the remaining outstanding stock of Monolith (“non-controlling interest”) at a time or times based on Monolith meeting certain technical and sales targets. During the first quarter of 2018, Monolith met the next set of technical and sales targets. As a result, and pursuant to the Securities Purchase Agreement, in April 2018 the Company acquired an additional 19% of the outstanding common stock of Monolith for $5.0 million , of which $4.0 million was paid to the stockholders of Monolith. On October 5, 2018, the Company acquired the remaining 19% outstanding common stock for $5.0 million . The additional investment, in the first quarter of 2017, resulted in the Company gaining control of Monolith and was accounted for as a step-acquisition with the fair value of the original investment immediately before the acquisition estimated to be approximately $3.5 million . As the fair value of the investment immediately prior to the transaction equaled the carrying value, there was no impact on the Company’s Consolidated Statements of Net Income. As the Securities Purchase Agreement includes an obligation of the Company to mandatorily redeem the non-controlling interest for cash, the fair value of the non-controlling interest was recognized as a liability on the Company’s Consolidated Balance Sheets. The original investment of $3.5 million , additional cash consideration of $14.2 million (net of cash acquired), and the non-cash consideration of the fair value of the commitment to purchase the non-controlling interest of $9.0 million resulted in a purchase price of $26.7 million . Changes in the fair value of the non-controlling interest are recognized in the Company’s Consolidated Statements of Net Income. Commencing March 1, 2017, Monolith was reflected as a consolidated subsidiary within the Company’s Consolidated Financial Statements. Had the acquisition occurred as of January 1, 2017, the impact on the Company’s consolidated results of operations would not have been material. The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the Monolith acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: Original investment $ 3,500 Cash, net of cash acquired 14,172 Non-cash, fair value of commitment to purchase non-controlling interest 9,000 Total purchase consideration $ 26,672 Allocation of consideration to assets acquired and liabilities assumed: Current assets, net $ 891 Property, plant, and equipment 789 Patented and unpatented technologies 6,720 Non-compete agreement 140 Goodwill 20,641 Current liabilities (639 ) Other non-current liabilities (1,870 ) $ 26,672 Included in Monolith’s current assets, net was approximately $0.7 million of receivables. All Monolith goodwill, other assets and liabilities were recorded in the Electronics segment and reflected in the United States geographic area. The goodwill resulting from this acquisition consists largely of the Company’s expected future product sales and synergies from combining Monolith’s products and technology with the Company’s existing electronics product portfolio. Goodwill for the above acquisition is not expected to be deductible for tax purposes. Pro Forma Results The following table summarizes, on a pro forma basis, the combined results of operations of the Company and IXYS as though the acquisition had occurred as of January 1, 2017. The Company has not included pro forma results of operations for U.S. Sensor or Monolith as these results were not material to the Company. The pro forma amounts presented are not necessarily indicative of either the actual consolidated results had the IXYS acquisition occurred as of January 1, 2017 or of future consolidated operating results. Three Months Ended (in thousands, except per share amounts) September 29, 2018 September 30, Net sales $ 439,191 $ 405,573 Income before income taxes 71,737 50,883 Net income 56,060 40,004 Net income per share — basic 2.23 1.61 Net income per share — diluted $ 2.18 $ 1.59 Nine Months Ended (in thousands, except per share amounts) September 29, September 30, Net sales $ 1,332,900 $ 1,171,283 Income before income taxes 228,503 102,429 Net income 179,264 95,047 Net income — basic 7.17 3.84 Net income — diluted $ 7.16 $ 3.78 Pro forma results presented above primarily reflect the following adjustments: Three Months Ended Nine Months Ended (in thousands) September 29, September 30, September 29, September 30, Amortization (a) $ 3,104 $ (6,304 ) $ 8,289 $ (18,905 ) Depreciation — 139 — 417 Transaction costs (b) — — 9,976 (9,976 ) Amortization of inventory step-up (c) — — 36,927 (36,927 ) Stock compensation (d) 421 (426 ) 5,110 (6,206 ) Interest expense (e) — (2,582 ) — (7,746 ) Income tax impact of above items $ (1,011 ) $ 2,906 $ (14,290 ) $ 25,802 (a) The amortization adjustment for the nine months ended September 29, 2018 primarily reflects the reduction of amortization expense in the period related to the Order backlog intangible asset. The Order backlog has a useful life of twelve months and will be fully amortized in the fiscal 2017 pro forma results. The amortization adjustment for the three and nine months ended September 30, 2017 reflects incremental amortization resulting for the measurement of intangibles at their fair values. (b) The transaction cost adjustments reflect the reversal of certain bank and attorney fees from the nine months ended September 29, 2018 and recognition of those fees during the nine months ended September 30, 2017 . (c) The amortization of inventory step-up adjustment reflects the reversal of the amount recognized during the nine months ended September 29, 2018 and the recognition of the full amortization during the nine months ended September 30, 2017 . The inventory step-up was amortized over five months as the inventory was sold. (d) The stock compensation adjustment reflects the reversal of the portion of stock compensation for IXYS stock options that were converted to Littelfuse stock options and expensed immediately during the nine months ended September 29, 2018 . The adjustment for the nine months ended September 30, 2017 reflect the incremental stock compensation for the converted stock options. (e) The interest expense adjustment reflects incremental interest expense related to the financing of the transaction.

Inventories

Inventories	9 Months Ended
	Sep. 29, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories The components of inventories at September 29, 2018 and December 30, 2017 are as follows: (in thousands) September 29 December 30 Raw materials $ 69,575 $ 39,030 Work in process 83,115 27,454 Finished goods 94,565 74,305 Total $ 247,255 $ 140,789

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	9 Months Ended
	Sep. 29, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets The amounts for goodwill and changes in the carrying value by segment for the nine months ended September 29, 2018 are as follows: (in thousands) Electronics Automotive Industrial Total As of December 30, 2017 $ 278,959 $ 135,829 $ 38,626 $ 453,414 Additions (a) 380,199 — — 380,199 Currency translation (1,090 ) (2,040 ) (129 ) (3,259 ) As of September 29, 2018 $ 658,068 $ 133,789 $ 38,497 $ 830,354 (a) The additions resulted from the acquisition of IXYS and an immaterial acquisition. The components of other intangible assets at September 29, 2018 are as follows: (in thousands, except weighted average useful life) Weighted Average Useful Life (Years) Gross Carrying Value Accumulated Amortization Net Book Value Patents, licenses and software 10.5 $ 192,215 $ 71,451 $ 120,764 Distribution network 12.6 44,098 34,214 9,884 Customer relationships, trademarks, and tradenames 18 312,267 69,478 242,789 Order backlog 0.3 12,420 8,706 3,714 Total $ 561,000 $ 183,849 $ 377,151 During the nine months ended September 29, 2018 , the Company recorded additions to other intangible assets of $212.7 million , related to the IXYS acquisition, the components of which were as follows: (in thousands, except weighted average useful life) Weighted Average Useful Life (Years) Amount Patents, licenses and software 8 $ 51,500 Customer relationships, trademarks, and tradenames 17.2 148,800 Order backlog 1 12,420 Total $ 212,720 During the three and nine months ended September 29, 2018 and September 30, 2017 , the Company recorded amortization expense of $13.1 million and $38.5 million and $6.3 million and $18.4 million , respectively, for intangible assets with definite lives. Estimated annual amortization expense related to intangible assets with definite lives as of September 29, 2018 is as follows: (in thousands) Amount 2018 $ 53,383 2019 40,454 2020 40,237 2021 38,403 2022 37,506 2023 and thereafter 206,586 Total $ 416,569

Debt

Debt	9 Months Ended
	Sep. 29, 2018
Debt Disclosure [Abstract]
Debt	Debt The carrying amounts of debt at September 29, 2018 and December 30, 2017 are as follows: (in thousands) September 29, December 30, Term Loan $ 155,000 $ 122,500 Euro Senior Notes, Series A due 2023 136,819 139,623 Euro Senior Notes, Series B due 2028 111,092 113,369 U.S. Senior Notes, Series A due 2022 25,000 25,000 U.S. Senior Notes, Series B due 2027 100,000 100,000 U.S. Senior Notes, Series A due 2025 50,000 — U.S. Senior Notes, Series B due 2030 125,000 — Other 2,694 — Unamortized debt issuance costs (4,892 ) (4,881 ) Total debt 700,713 495,611 Less: Current maturities (10,076 ) (6,250 ) Total long-term debt $ 690,637 $ 489,361 Revolving Credit Facility / Term Loan On March 4, 2016, the Company entered into a five -year credit agreement (“Credit Agreement”) with a group of lenders for up to $700.0 million . The Credit Agreement consisted of an unsecured revolving credit facility (“Revolving Credit Facility”) of $575.0 million and an unsecured term loan credit facility (“Term Loan”) of up to $125.0 million . In addition, the Company had the ability, from time to time, to increase the size of the Revolving Credit Facility and the Term Loan by up to an additional $150.0 million , in the aggregate, in each case in minimum increments of $25.0 million , subject to certain conditions and the agreement of participating lenders. On October 13, 2017, the Company amended the Credit Agreement to increase the Revolving Credit Facility from $575.0 million to $700.0 million and increase the Term Loan from $125.0 million to $200.0 million and to extend the expiration date from March 4, 2021 to October 13, 2022. The Credit Agreement also includes the option for the Company to increase the size of the Revolving Credit Facility and the Term Loan by up to an additional $300.0 million , in the aggregate, subject to the satisfaction of certain conditions set forth in the Credit Agreement. Term Loans may be made in up to two advances. The first advance of $125.0 million occurred on October 13, 2017 and the second advance of $75.0 million occurred on January 16, 2018. For the Term Loan, the Company is required to make quarterly principal payments of 1.25% of the original term loan ( $2.5 million with the second advance on January 16, 2018) through maturity, with the remaining balance due on October 13, 2022. In addition to the quarterly principal payments, the Company paid an additional $35.0 million of principal on the term loan during the nine months ended September 29, 2018 . Outstanding borrowings under the Credit Agreement bear interest, at the Company’s option, at either LIBOR, fixed for interest periods of one , two , three or six -month periods, plus 1.00% to 2.00% , or at the bank’s Base Rate, as defined, plus 0.00% to 1.00% , based upon the Company’s Consolidated Leverage Ratio, as defined. The Company is also required to pay commitment fees on unused portions of the credit agreement ranging from 0.15% to 0.25% , based on the Consolidated Leverage Ratio, as defined in the agreement. The credit agreement includes representations, covenants and events of default that are customary for financing transactions of this nature. The effective interest rate on outstanding borrowings under the credit facility was 3.49% at September 29, 2018 . As of September 29, 2018 , the Company had $0.1 million outstanding in letters of credit and had available $699.9 million of borrowing capacity under the Revolving Credit Facility. At September 29, 2018 , the Company was in compliance with all covenants under the Credit Agreement. Senior Notes On December 8, 2016, the Company entered into a Note Purchase Agreement, pursuant to which the Company issued and sold €212 million aggregate principal amount of senior notes in two series. The funding date for the Euro denominated senior notes occurred on December 8, 2016 for €117 million in aggregate amount of 1.14% Senior Notes, Series A, due December 8, 2023 (“Euro Senior Notes, Series A due 2023”), and €95 million in aggregate amount of 1.83% Senior Notes, Series B due December 8, 2028 (“Euro Senior Notes, Series B due 2028”) (together, the “Euro Senior Notes”). Interest on the Euro Senior Notes is payable semiannually on June 8 and December 8, commencing June 8, 2017. On December 8, 2016, the Company entered into a Note Purchase Agreement, pursuant to which the Company issued and sold $125 million aggregate principal amount of senior notes in two series. On February 15, 2017, $25 million in aggregate principal amount of 3.03% Senior Notes, Series A, due February 15, 2022 (“U.S. Senior Notes, Series A due 2022”), and $100 million in aggregate principal amount of 3.74% Senior Notes, Series B, due February 15, 2027 (“U.S. Senior Notes, Series B due 2027”) (together, the “U.S. Senior Notes due 2022 and 2027”) were funded. Interest on the U.S. Senior Notes due 2022 and 2027 is payable semiannually on February 15 and August 15, commencing August 15, 2017. On November 15, 2017, the Company entered into a Note Purchase Agreement pursuant to which the Company issued and sold $175 million in aggregate principal amount of senior notes in two series. On January 16, 2018, $50 million aggregate principal amount of 3.48% Senior Notes, Series A, due February 15, 2025 (“U.S. Senior Notes, Series A due 2025”) and $125 million in aggregate principal amount of 3.78% Senior Notes, Series B, due February 15, 2030 (“U.S. Senior Notes, Series B due 2030”) (together the “U.S. Senior Notes due 2025 and 2030” and with the Euro Senior Notes and the U.S. Senior Notes due 2022 and 2027, the “Senior Notes”) were funded. Interest on the U.S. Senior Notes due 2025 and 2030 is payable semiannually on February 15 and August 15, commencing on August 15, 2018. The Senior Notes have not been registered under the Securities Act, or applicable state securities laws. The Senior Notes are general unsecured senior obligations and rank equal in right of payment with all existing and future unsecured unsubordinated indebtedness of the Company. The Senior Notes are subject to certain customary covenants, including limitations on the Company’s ability, with certain exceptions, to engage in mergers, consolidations, asset sales and transactions with affiliates, to engage in any business that would substantially change the general business of the Company, and to incur liens. In addition, the Company is required to satisfy certain financial covenants and tests relating to, among other matters, interest coverage and leverage. At September 29, 2018 , the Company was in compliance with all covenants under the Senior Notes. The Company may redeem the Senior Notes upon the satisfaction of certain conditions and the payment of a make-whole amount to noteholders, and are required to offer to repurchase the Senior Notes at par following certain events, including a change of control.

Fair Value of Assets and Liabil

Fair Value of Assets and Liabilities	9 Months Ended
	Sep. 29, 2018
Fair Value Disclosures [Abstract]
Fair Value of Assets and Liabilities	Fair Value of Assets and Liabilities For assets and liabilities measured at fair value on a recurring and nonrecurring basis, a three-level hierarchy of measurements based upon observable and unobservable inputs is used to arrive at fair value. Observable inputs are developed based on market data obtained from independent sources, while unobservable inputs reflect the Company’s assumptions about valuation based on the best information available in the circumstances. Depending on the inputs, the Company classifies each fair value measurement as follows: Level 1 —Valuations based on unadjusted quoted prices for identical assets or liabilities in active markets; Level 2 —Valuations based upon quoted prices for similar instruments, prices for identical or similar instruments in markets that are not active, or model-derived valuations, all of whose significant inputs are observable, and Level 3 —Valuations based upon one or more significant unobservable inputs. Following is a description of the valuation methodologies used for instruments measured at fair value and their classification in the valuation hierarchy. Investments in Equity Securities Investments in equity securities listed on a national market or exchange are valued at the last sales price and classified within Level 1 of the valuation hierarchy and recorded in investments and other assets. Mutual Funds The Company has a non-qualified Supplemental Retirement and Savings Plan which provides additional retirement benefits for certain management employees and named executive officers by allowing participants to defer a portion of their annual compensation. The Company maintains accounts for participants through which participants make investment elections. The marketable securities are classified as Level 1 under the fair value hierarchy as they are maintained in mutual funds with readily determinable fair value and recorded in other assets. There were no changes during the quarter ended September 29, 2018 to the Company’s valuation techniques used to measure asset and liability fair values on a recurring basis. As of September 29, 2018 , and December 30, 2017 , the Company did not hold any non-financial assets or liabilities that are required to be measured at fair value on a recurring basis. The following table presents assets measured at fair value by classification within the fair value hierarchy as of September 29, 2018 : Fair Value Measurements Using (in thousands) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Investments in equity securities $ 12,204 $ — $ — $ 12,204 Mutual funds 10,546 — — 10,546 The following table presents assets measured at fair value by classification within the fair value hierarchy as of December 30, 2017 : Fair Value Measurements Using (in thousands) Quoted Prices in Significant Significant Total Investments in equity securities $ 10,993 $ — $ — $ 10,993 Mutual funds 7,962 — — 7,962 In addition to the methods and assumptions used for the financial instruments recorded at fair value as discussed above, the following methods and assumptions are used to estimate the fair value of other financial instruments that are not marked to market on a recurring basis. The Company’s other financial instruments include cash and cash equivalents, short-term investments, accounts receivable and its long-term debt. Due to their short-term maturity, the carrying amounts of cash and cash equivalents, short-term investments and accounts receivable approximate their fair values. The Company’s revolving and term loan debt facilities’ fair values approximate book value at September 29, 2018 and December 30, 2017 , as the rates on these borrowings are variable in nature. The carrying value and estimated fair values of the Company’s Euro Senior Notes, Series A and Series B and USD Senior Notes, Series A and Series B, as of September 29, 2018 and December 30, 2017 were as follows: September 29, 2018 December 30, 2017 (in thousands) Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Euro Senior Notes, Series A due 2023 $ 136,819 $ 135,392 $ 139,623 $ 138,294 Euro Senior Notes, Series B due 2028 111,092 108,511 113,369 111,579 USD Senior Notes, Series A due 2022 25,000 24,052 25,000 24,737 USD Senior Notes, Series B due 2027 100,000 94,474 100,000 99,992 USD Senior Notes, Series A due 2025 50,000 47,333 — — USD Senior Notes, Series B due 2030 125,000 115,231 — —

Benefit Plans

Benefit Plans	9 Months Ended
	Sep. 29, 2018
Retirement Benefits [Abstract]
Benefit Plans	Benefit Plans The Company has company-sponsored defined benefit pension plans covering employees in the U.K., Germany, the Philippines, China, Japan, and France. The amount of the retirement benefits provided under the plans is based on years of service and final average pay. The Company recognizes interest cost, expected return on plan assets, and amortization of prior service, net within Other expense (income), net in the Condensed Consolidated Statements of Net Income. The components of net periodic benefit cost for the three and nine months ended September 29, 2018 and September 30, 2017 were as follows: For the Three Months Ended For the Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Components of net periodic benefit cost: Service cost $ 533 $ 408 $ 1,599 $ 1,224 Interest cost 501 360 1,503 1,080 Expected return on plan assets (540 ) (476 ) (1,620 ) (1,428 ) Amortization of prior service 74 84 222 252 Net periodic benefit cost $ 568 $ 376 $ 1,704 $ 1,128 The Company expects to make approximately $2.3 million of cash contributions to its pension plans in 2018.

Shareholders' Equity

Shareholders' Equity	9 Months Ended
	Sep. 29, 2018
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity The following table sets forth the changes in shareholders’ equity for the nine months ended September 29, 2018 : (in thousands) Littelfuse, Inc. Shareholders’ Equity Non- controlling Interest Total Balance at December 30, 2017 927,419 137 927,556 Net income 131,900 — 131,900 Other comprehensive loss (24,168 ) — (24,168 ) Stock-based compensation 23,153 — 23,153 Withheld shares on restricted share units for withholding taxes (7,252 ) — (7,252 ) Stock options exercised 25,171 — 25,171 Issuance of common stock (a) 472,301 — 472,301 Cash dividends paid (29,258 ) — (29,258 ) Non-controlling interest — (7 ) (7 ) Balance at September 29, 2018 1,519,266 130 1,519,396 (a) The issuance of common stock ( 2,092,491 shares) during the nine months ended September 29, 2018 relates to the acquisition of IXYS. See Note 2, Acquisitions for further discussion. As of October 29, 2018, the Company has repurchased 200,000 shares of its common stock totaling $35.9 million since the quarter ended September 29, 2018.

Other Comprehensive Loss (Incom

Other Comprehensive Loss (Income)	9 Months Ended
	Sep. 29, 2018
Equity [Abstract]
Other Comprehensive Loss (Income)	Other Comprehensive (Loss) Income Change in other comprehensive (loss) income by component were as follows: (in thousands) Three Months Ended September 29, 2018 Three Months Ended September 30, 2017 Pre-tax Tax Net of Tax Pre-tax Tax Net of Tax Defined benefit pension plans adjustments $ (115 ) $ — $ (115 ) $ (149 ) $ 89 $ (60 ) Unrealized loss on investments — — — (1,710 ) — (1,710 ) Foreign currency translation adjustments (7,832 ) — (7,832 ) 1,531 — 1,531 Total change in other comprehensive (loss) income $ (7,947 ) $ — $ (7,947 ) $ (328 ) $ 89 $ (239 ) (in thousands) Nine Months Ended September 29, 2018 Nine Months Ended September 30, 2017 Pre-tax Tax Net of Tax Pre-tax Tax Net of Tax Defined benefit pension plans adjustments $ 630 $ (18 ) $ 648 $ (585 ) $ 144 $ (441 ) Unrealized loss on investments — — — (1,235 ) — (1,235 ) Foreign currency translation adjustments (24,816 ) — (24,816 ) 2,912 — 2,912 Total change in other comprehensive (loss) income $ (24,186 ) $ (18 ) $ (24,168 ) $ 1,092 $ 144 $ 1,236 The following table sets forth the changes in accumulated other comprehensive (loss) income by component for the nine months ended September 29, 2018 : (in thousands) Pension and postretirement liability and reclassification adjustments Unrealized gain (loss) on investments Foreign currency translation adjustment Accumulated other comprehensive income (loss) Balance at December 30, 2017 $ (10,836 ) $ 9,795 $ (62,627 ) $ (63,668 ) Cumulative effect adjustment (a) — (9,795 ) — (9,795 ) Activity in the period 648 — (24,816 ) (24,168 ) Balance at September 29, 2018 $ (10,188 ) $ — $ (87,443 ) $ (97,631 ) (a) The Company adopted ASU 2016-01 on December 31, 2017 on a modified retrospective basis, recognizing the cumulative effect as a $9.8 million increase to retained earnings. See Note 1, Summary of Significant Accounting Policies and Other Information , for further discussion. Amounts reclassified from accumulated other comprehensive (loss) income to earnings for the three and nine months ended September 29, 2018 and the three and nine months September 30, 2017 were as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Pension and Postemployment plans: Amortization of prior service $ 74 $ 84 $ 222 $ 252 The Company recognizes net periodic pension cost, which includes amortization of prior service costs, in both selling, general, and administrative expenses and cost of sales within the Condensed Consolidated Statements of Net Income, depending on the functional area of the underlying employees included in the plans.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 29, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The effective tax rates for the three and nine months ended September 29, 2018 were 21.5% and 20.1% , respectively, compared to effective tax rates for the three and nine months ended September 30, 2017 of 22.9% and 18.7% , respectively. The effective tax rate for the third quarter of 2018 was lower than the effective tax rate for the third quarter of 2017 primarily due to a third quarter 2017 accrual for taxes on certain undistributed earnings of non-US affiliates, offset in part by the impact of US tax reform and the acquisition of IXYS. The effective tax rate of the first nine months of 2018 is higher than the effective tax rates for the first nine months of 2017 primarily due to the impact of U.S. tax reform and the acquisition of IXYS. The effective tax rates for the 2017 periods were lower than the then applicable 35% U.S. statutory tax rate primarily due to income earned in lower tax jurisdictions. On December 22, 2017, the U.S. enacted legislation commonly referred to as the Tax Cuts and Jobs Act (the "Tax Act"). Among other things, the Tax Act reduces the U.S. corporate federal income tax rate from 35% to 21%, adds base broadening provisions which limit deductions and address excessive international tax planning, imposes a one-time tax (the “Toll Charge”) on accumulated earnings of certain non-U.S. subsidiaries and enables repatriation of earnings of non-U.S. subsidiaries free of U.S. federal income tax. Other than the Toll Charge (which, except for the IXYS impact, is applicable to the Company for 2017), the provisions will generally be applicable to the Company in 2018 and beyond. In accordance with the guidance provided in SEC Staff Accounting Bulletin (“SAB”) No. 118, in the fourth quarter of 2017 the Company recorded a charge of $47.0 million as a provisional reasonable estimate of the impact of the Tax Act, including $49.0 million for the Toll Charge net of $2.0 million for other net tax benefits. The Company did not adjust the provisional reasonable estimate in the first nine months of 2018 . In addition, the Company recorded a preliminary estimate of $8.0 million for the Toll Charge associated with IXYS as part of the IXYS acquisition purchase price allocation (this estimate reflects a reduction of $2.0 million recorded in the third quarter of 2018 ). This estimate was reflected in the opening balance sheet as an increase to goodwill and other long-term liabilities. The Company is continuing to analyze the Tax Act and plans to finalize the 2017 provisional reasonable estimate within the measurement period outlined in SAB No. 118 and the IXYS Toll Charge estimate during the purchase price allocation period. The final charges may differ from the estimates if provisions of the Tax Act, and their interaction with other provisions of the U.S. Internal Revenue Code, are interpreted differently than interpretations made by the Company in determining the estimates, whether through issuance of administrative guidance, or through further review of the Tax Act by the Company and its advisors. Aside from these interpretation issues, the final charges may differ from the estimates due to refinements of accumulated non-U.S. earnings and tax pool data. The Company has elected to pay the 2017 Littelfuse Toll Charge and will elect to pay the 2018 IXYS Toll Charge over the eight-year period prescribed by the Tax Act. The long-term portion of these Toll Charges totaling $29.7 million (which includes the 2017 Littelfuse provisional reasonable estimate and the 2018 IXYS provisional estimate, partially offset by foreign tax credits, other current deductions and the actual 2018 and anticipated 2019 annual installment payments) is recorded in the Other long-term liabilities on the Condensed Consolidated Balance Sheet as of September 29, 2018 . The Company recognized deferred tax liabilities of $12.0 million ( $11.8 million for non-U.S. taxes and $0.2 million for U.S. state taxes) as of December 30, 2017 related to taxes on certain non-U.S. earnings which are not considered to be permanently reinvested. Some of these taxes may provide a U.S. federal income tax benefit as a foreign tax credit. However, due to uncertainty in regard to the Tax Act’s provisions, no such tax benefit was recorded as of December 30, 2017 , and no such tax benefit was recorded in the first nine months of 2018. The Company will reconsider this provisional conclusion when it finalizes its 2017 preliminary reasonable estimate of the impact of the Tax Act, based upon interpretations and administrative guidance as of that time. One of the base broadening provisions of the Tax Act is commonly referred to as the “GILTI” provisions. In accordance with guidance issued by the FASB staff, the Company has not adopted an accounting policy for GILTI. Thus, deferred taxes were computed without consideration of the possible future impact of the GILTI provisions. The Company intends to adopt an accounting policy for GILTI within the measurement period outlined in SAB 118. Although such an accounting policy has not been adopted, the Company considered GILTI when determining the current portion of income tax expense recorded for the three and nine months ended September 29, 2018 . Although certain administrative guidance has been issued, the appropriate application of many provisions of the Tax Act remain uncertain. The Company used its best judgement as to the application of these provisions in determining its income tax expense for the three and nine months ended September 29, 2018 . Adjustments to income tax expense related to the first nine months of 2018 may be necessary if provisions of the Tax Act, and their interaction with other provisions of the U.S. Internal Revenue Code, are interpreted differently than interpretations made by the Company in determining its estimates, whether through issuance of administrative guidance, or through further review of the Tax Act by the Company and its advisors.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 29, 2018
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share The following table sets forth the computation of basic and diluted earnings per share: Three Months Ended Nine Months Ended (in thousands, except per share amounts) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Numerator: Net income as reported 53,546 42,808 131,900 130,338 Denominator: Weighted average shares outstanding Basic 25,109 22,713 24,817 22,678 Effect of dilutive securities 362 240 395 228 Diluted 25,471 22,953 25,212 22,906 Earnings Per Share: Basic earnings per share 2.13 1.88 5.31 5.75 Diluted earnings per share 2.10 1.87 5.23 5.69 Potential shares of common stock relating to stock options excluded from the earnings per share calculation because their effect would be anti-dilutive were 38,082 and 30,579 for the three months ended September 29, 2018 and September 30, 2017 , respectively, and 39,446 and 35,858 for the nine months ended September 29, 2018 and September 30, 2017 , respectively.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 29, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions As a result of the Company’s acquisition of IXYS, the Company has equity ownerships in various investments that are accounted for under the equity method. The following is a description of the investments and related party transactions. Powersem GmbH: The Company owns 45% of the outstanding equity of Powersem GmbH (“Powersem”), a module manufacturer based in Germany. During the three and nine months ended September 29, 2018 , the Company recorded revenues of $0.2 million and $0.6 million , respectively from sales of products to Powersem for use as components in their products. During the three and nine months ended September 29, 2018 , the Company purchased $1.3 million and $3.4 million , respectively of products from Powersem. At September 29, 2018 , the accounts receivable balance from Powersem was $0.1 million and the accounts payable balance to Powersem was $0.2 million . EB Tech Ltd.: The Company owns approximately 20% of the outstanding equity of EB Tech Ltd. (“EB Tech”), a company with expertise in radiation technology based in South Korea. During the three and nine months ended September 29, 2018 EB Tech rendered processing services for the Company totaling approximately $0.1 million and $0.3 million , respectively. As of September 29, 2018 , the Company’s accounts payable balance to EB Tech was $0.1 million . Automated Technology, Inc. : The Company owns approximately 24% of the outstanding common shares of Automated Technology, Inc (“ATEC”), a supplier located in the Philippines that provides assembly and test services. During the three and nine months ended September 29, 2018 , ATEC rendered assembly and test services to the Company totaling approximately $2.5 million and $7.7 million , respectively. As of September 29, 2018 , the Company’s accounts payable balance to ATEC was $0.6 million .

Segment Information

Segment Information	9 Months Ended
	Sep. 29, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information The Company and its subsidiaries design, manufacture and sell components and modules for circuit protection, power control and sensing throughout the world. The Company reports its operations by the following segments: Electronics, Automotive, and Industrial. An operating segment is defined as a component of an enterprise that engages in business activities from which it may earn revenues and incur expenses, and about which separate financial information is regularly evaluated by the Chief Operating Decision Maker (“CODM”) in deciding how to allocate resources. The CODM is the Company’s President and Chief Executive Officer (“CEO”). The CODM allocates resources to and assesses the performance of each operating segment using information about its revenue and operating income (loss) before interest and taxes, but does not evaluate the operating segments using discrete balance sheet information. Sales, marketing, and research and development expenses are charged directly into each operating segment. Manufacturing, purchasing, logistics, customer service, finance, information technology, and human resources are shared functions that are allocated back to the three operating segments. The Company does not report inter-segment revenue because the operating segments do not record it. Certain expenses, determined by the CODM to be strategic in nature and not directly related to segments current results, are not allocated but identified as “Other”. Additionally, the Company does not allocate interest and other income, interest expense, or taxes to operating segments. These costs are not allocated to the segments, as management excludes such costs when assessing the performance of the segments. Although the CODM uses operating income (loss) to evaluate the segments, operating costs included in one segment may benefit other segments. Except as discussed above, the accounting policies for segment reporting are the same as for the Company as a whole. • Electronics Segment : Consists of one of the broadest product offerings in the industry, including fuses and fuse accessories, positive temperature coefficient (“PTC”) resettable fuses, polymer electrostatic discharge (“ESD”) suppressors, varistors, gas discharge tubes; semiconductor and power semiconductor products such as discrete transient voltage suppressor (“TVS”) diodes, TVS diode arrays, protection and switching thyristors, silicon carbide, metal-oxide-semiconductor field-effect transistors (“MOSFETs”) and silicon carbide diodes; and insulated gate bipolar transistors (“IGBT”) technologies. The segment covers a broad range of end markets, including automotive electronics, industrial applications, data and telecommunications, medical devices, consumer electronics and appliances. • Automotive Segment: Consists of a wide range of circuit protection, power control and sensing technologies for global original equipment manufacturers (“OEMs”), Tier-I suppliers and parts distributors in the automotive, commercial vehicle, and agricultural and construction equipment industries. Passenger car fuse products include fuses and fuse accessories for internal combustion engine vehicles and hybrid and electric vehicles, which are blade fuses, battery cable protectors, varistors, high-current fuses, and high-voltage fuses. Commercial vehicle products include fuses, switches, relays, and power distribution modules for the commercial vehicle industry. Automotive sensor products include a wide range of automotive and commercial vehicle sensors designed to monitor the passenger compartment occupants and environment as well as the vehicle’s powertrain, emissions, speed and suspension. • Industrial Segment: Consists of power fuses, protection relays and controls and other circuit protection products for use in heavy industrial applications such as mining, oil and gas, energy storage, construction, HVAC systems, elevator and other industrial equipment. Segment information is summarized as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Net sales Electronics $ 296,472 $ 175,899 $ 860,240 $ 499,052 Automotive 114,416 113,797 367,718 338,094 Industrial 28,303 28,193 88,229 79,539 Total net sales $ 439,191 $ 317,889 $ 1,316,187 $ 916,685 Depreciation and amortization Electronics $ 15,898 $ 8,986 $ 45,227 $ 26,080 Automotive 5,891 5,622 17,830 16,572 Industrial 1,364 1,338 4,291 3,982 Other 3,105 — 8,712 — Total depreciation and amortization $ 26,258 $ 15,946 $ 76,060 $ 46,634 Operating income (loss) Electronics $ 72,464 $ 44,345 $ 193,739 $ 122,518 Automotive 10,863 16,821 44,965 47,599 Industrial 4,134 3,757 14,123 5,769 Other (a) (11,233 ) (6,314 ) (79,406 ) (8,155 ) Total operating income $ 76,228 $ 58,609 $ 173,421 $ 167,731 Interest expense 5,775 3,467 16,980 9,868 Foreign exchange loss (gain) 982 632 (6,372 ) (1,483 ) Other expense (income), net 1,259 (1,013 ) (2,362 ) (962 ) Income before income taxes $ 68,212 $ 55,523 $ 165,175 $ 160,308 (a) Included in “Other” Operating income (loss) for the 2018 third quarter is $11.2 million ( $79.4 million year-to-date) of charges primarily related to IXYS acquisition, which include $36.9 million year-to-date of purchase accounting inventory step-up charges previously recorded during the first and second quarters of 2018, $2.5 million ( $16.3 million year-to-date) in acquisition-related and integration costs primarily related to legal, accounting and other expenses, $3.1 million ( $8.7 million year-to-date) in backlog amortization costs, $4.6 million ( $9.4 million year-to-date) of severance and other restructuring charges, and $4.5 million year-to-date stock compensation expense recognized immediately upon close for converted IXYS options related to prior services periods. In addition, there were $1.0 million ( $3.6 million year-to-date) of severance, other restructuring charges and acquisition-related expenses for other contemplated acquisitions which included $1.1 million year-to-date associated with the exit of the Custom business in the second quarter. The third quarter of 2017 includes approximately $6.3 million of non-segment charges ( $8.2 million year-to-date). These charges were primarily attributable to acquisition-related and integration costs related to legal, accounting and other expenses associated with completed or contemplated acquisitions of approximately $4.8 million ( $6.6 million year-to-date), including $1.6 million ( $1.6 million year-to-date) of purchase accounting inventory charges related to the Company's 2017 acquisition of U.S. Sensor and $1.5 million ( $1.5 million year-to-date) of charges related to restructuring and production transfers in our Asia operations. The Company’s net sales by country are as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Net sales United States $ 125,867 $ 103,233 $ 386,980 $ 290,538 China (a) 117,813 82,986 352,097 240,731 Other countries (b) 195,511 131,670 577,110 385,416 Total net sales $ 439,191 $ 317,889 $ 1,316,187 $ 916,685 (a) Includes mainland China, Taiwan, and Hong Kong. (b) Each country included in other countries are less than 10% of net sales. The Company’s long-lived assets by country, as of September 29, 2018 and December 30, 2017 , were as follows: (in thousands) September 29, December 30, Long-lived assets United States $ 66,098 $ 23,490 China (a) 89,856 86,866 Mexico 69,395 62,510 Germany 37,281 1,082 Philippines 31,589 31,129 Other countries 49,664 45,500 Total long-lived assets $ 343,883 $ 250,577 (a) Includes mainland China, Taiwan, and Hong Kong. The Company’s additions to long-lived assets by country were as follows: Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 Additions to long-lived assets United States $ 5,636 $ 2,752 China (a) 19,043 22,165 Mexico 14,089 15,041 Germany 5,917 67 Philippines 6,133 2,018 Other countries 5,128 6,427 Total additions to long-lived assets $ 55,946 $ 48,470 (a) Includes mainland China, Taiwan, and Hong Kong.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies and Other Information (Policies)	9 Months Ended
	Sep. 29, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company’s accompanying unaudited Condensed Consolidated Financial Statements have been prepared in accordance with U.S. Generally Accepted Accounting Principles (“GAAP”) for interim financial information, the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, certain information and disclosures normally included in the consolidated balance sheets, statements of net income and comprehensive income and cash flows prepared in conformity with U.S. GAAP have been condensed or omitted as permitted by such rules and regulations, although the Company believes that the disclosures made are adequate to make the information not misleading. They have been prepared in accordance with accounting policies described in the Company’s Annual Report on Form 10-K for the year ended December 30, 2017 , which should be read in conjunction with the disclosures therein. In the opinion of management, all adjustments considered necessary for a fair presentation have been included and are of a normal, recurring nature. Operating results for interim periods are not necessarily indicative of annual operating results.
Revenue Recognition	Revenue Recognition Adoption On December 31, 2017, the Company adopted new guidance on revenue from contracts with customers using the modified retrospective method. The adoption did not have a significant impact on the Company’s consolidated financial statements. Revenue Recognition The Company recognizes revenue on product sales in the period in which the Company satisfies its performance obligation and control of the product is transferred to the customer. The Company’s sales arrangements with customers are predominately short term in nature and generally provide for transfer of control at the time of shipment as this is the point at which title and risk of loss of the product transfers to the customer. At the end of each period, for those shipments where title to the products and the risk of loss and rewards of ownership do not transfer until the product has been received by the customer, the Company adjusts revenues and cost of sales for the delay between the time that the products are shipped and when they are received by the customer. The amount of revenue recorded reflects the consideration to which the Company expects to be entitled in exchange for goods and may include adjustments for customer allowance, rebates and price adjustments. The Company’s distribution channels are primarily through direct sales and independent third-party distributors. The Company has elected the practical expedient under Accounting Standards Codification ("ASC") 340-40-25-4 to expense commissions when incurred as the amortization period of the commission asset the Company would have otherwise recognized is less than one year. Revenue and Billing The Company generally accepts orders from customers through receipt of purchase orders or electronic data interchange based on written sales agreements and purchasing contracts. Contract pricing and selling agreement terms are based on market factors, costs, and competition. Pricing is often negotiated as an adjustment (premium or discount) from the Company’s published price lists. The customer is invoiced when the Company’s products are shipped to them in accordance with the terms of the sales agreement. As the Company’s standard payment terms are less than one year, the Company has elected the practical expedient under ASC 606-10-32-18 to not assess whether a contract has a significant financing component. The Company also elected the practical expedient provided in ASC 606-10-25-18B to treat all product shipping and handling activities as fulfillment activities, and therefore recognize the gross revenue associated with the contract, inclusive of any shipping and handling revenue. This is similar to the Company’s prior practice and therefore the effect of the new guidance is immaterial. Ship and Debit Program Some of the terms of the Company’s sales agreements and normal business conditions provide customers (distributors) the ability to receive price adjustments on products previously shipped and invoiced. This practice is common in the industry and is referred to as a “ship and debit” program. This program allows the distributor to debit the Company for the difference between the distributors’ contracted price and a lower price for specific transactions. Under certain circumstances (usually in a competitive situation or large volume opportunity), a distributor will request authorization for pricing allowances to reduce its price. When the Company approves such a reduction, the distributor is authorized to “debit” its account for the difference between the contracted price and the lower approved price. The Company establishes reserves for this program based on historic activity and actual authorizations for the debit and recognizes these debits as a reduction of revenue. Return to Stock The Company has a return to stock policy whereby certain customers, with prior authorization from Littelfuse management, can return previously purchased goods for full or partial credit. The Company establishes an estimated allowance for these returns based on historic activity. Sales revenue and cost of sales are reduced to anticipate estimated returns. Volume Rebates The Company offers volume based sales incentives to certain customers to encourage greater product sales. If customers achieve their specific quarterly or annual sales targets, they are entitled to rebates. The Company estimates the projected amount of rebates that will be achieved by the customer and recognizes this estimated cost as a reduction to revenue as products are sold.
Recently Adopted and Issued Accounting Standards	Recently Adopted Accounting Standards In March 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2017-07 “Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post-Retirement Benefit Cost,” which changed the presentation of net periodic pension and post-retirement benefit cost (net benefit cost) within the Statement of Income. Under the previous guidance, net benefit cost was reported as an employee cost within operating income. The amendment required the bifurcation of net benefit cost, with the service cost component to be presented with other employee compensation costs in operating income while the other components will be reported separately outside of income from operations. ASU No. 2017-07 was effective for the first quarter of 2018 with the Company adopting the new standard on December 31, 2017. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Recognition and Measurement of Financial Assets and Financial Liabilities” which addressed certain aspects of the recognition, measurement, presentation and disclosure of financial instruments. The ASU requires the Company to recognize any changes in the fair value of certain equity investments in net income. Previously these changes were recognized in other comprehensive income ("OCI"). The Company adopted the new standard on December 31, 2017, on a modified retrospective basis, recognizing the cumulative effect as a $9.8 million increase to retained earnings. As a result of the adoption of the new standard and change in fair value of our equity investments, for the nine months ended September 29, 2018 , the Company recognized an unrealized loss of $0.3 million in Other (income) expense, net in the Condensed Consolidated Statements of Net Income. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers” (Topic 606) which supersedes the revenue recognition requirements in ASC 605, “Revenue Recognition.” This ASU provides a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most current revenue recognition guidance. The guidance permits two implementation approaches, one requiring retrospective application of the new standard with restatement of prior years and one requiring prospective application of the new standard with disclosure of results under old standards. The Company adopted the new standard on December 31, 2017 using the modified retrospective method, however, no adjustment to retained earnings was needed. The new guidance did not have a material effect on the Company’s Condensed Consolidated Statements of Net Income. See the Revenue Recognition section above for further discussion. In October 2016, the FASB issued ASU No. 2016-16, "Income Taxes” (Topic 740). This ASU update requires entities to recognize the income tax consequences of many intercompany asset transfers at the transaction date. The seller and buyer will immediately recognize the current and deferred income tax consequences of an intercompany transfer of an asset other than inventory. The tax consequences were previously deferred. The Company adopted the new standard on December 31, 2017 and it did not have a material impact. Recently Issued Accounting Standards In February 2016, the FASB issued ASU No. 2016-02, "Leases" (Topic 842). This ASU requires lessees to recognize, on the balance sheet, assets and liabilities for the rights and obligations created by leases of greater than twelve months. The accounting by lessors will remain largely unchanged. The ASU is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted. Adoption will require a modified retrospective transition. The Company will adopt the standard in the first quarter of 2019. The Company has made progress on assessing the Company’s portfolio of leases and compiling a central repository of all active leases. We are in the process of assessing the design of the future lease process and drafting a policy to address the new standard requirements. Key lease data elements are being evaluated including developing a methodology for determining the incremental borrowing rate across all countries where we have operations. While the Company has not yet completed its evaluation of the impact the new lease accounting standard will have on its Consolidated Financial Statements, the Company expects to recognize right of use assets and lease liabilities for its operating leases in the Consolidated Balance Sheet upon adoption. In January 2018, the FASB released guidance on the accounting for tax on the global intangible low-taxed income ("GILTI") provisions of the 2017 U.S. Tax Cuts and Jobs Act (the “Tax Act”). The GILTI provisions impose a tax on foreign income in excess of a deemed return on tangible assets of foreign corporations. The guidance indicates that either accounting for deferred taxes related to GILTI inclusions or treating any taxes on GILTI inclusions as period cost are both acceptable methods subject to an accounting policy election. The Company has not yet completed its assessment and therefore has not yet elected an accounting policy. In February 2018, the FASB issued ASU No. 2018-02 “Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income,” which permits the reclassification of tax effects stranded in accumulated other comprehensive income to retained earnings as a result of the Tax Act. The standard also requires entities to disclose whether or not they elected to reclassify the tax effects related to the Tax Act as well as their policy for releasing income tax effects from accumulated other comprehensive income. The standard allows the option of applying either a retrospective adoption, meaning the standard is applied to all periods in which the effect of the Tax Act is recognized, or applying the amendments in the period of adoption, meaning an adjustment is made to shareholder’s equity as of the beginning of the reporting period. ASU 2018-02 will be effective in the first quarter of 2019; however early adoption is permitted for interim and annual periods, including the reporting period in which the Tax Act was enacted. The Company is currently evaluating the impact of ASU 2018-02 on the Consolidated Financial Statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies and Other Information (Tables)	9 Months Ended
	Sep. 29, 2018
Accounting Policies [Abstract]
Revenue Disaggregation	The following table disaggregates the Company’s revenue by primary business units for the three and nine months ended September 29, 2018 : Three Months Ended September 29, 2018 (in thousands) Electronics Segment Automotive Segment Industrial Segment Total Electronics – Passive Products and Sensors $ 124,174 $ — $ — $ 124,174 Electronics – Semiconductor 172,298 — — 172,298 Passenger Car Products — 57,761 — 57,761 Automotive Sensors — 27,311 — 27,311 Commercial Vehicle Products — 29,344 — 29,344 Industrial Products — — 28,303 28,303 Total $ 296,472 $ 114,416 $ 28,303 $ 439,191 Nine Months Ended September 29, 2018 (in thousands) Electronics Segment Automotive Segment Industrial Segment Total Electronics – Passive Products and Sensors $ 366,990 $ — $ — $ 366,990 Electronics – Semiconductor 493,250 — — 493,250 Passenger Car Products — 184,922 — 184,922 Automotive Sensors — 89,362 — 89,362 Commercial Vehicle Products — 93,434 — 93,434 Industrial Products — — 88,229 88,229 Total $ 860,240 $ 367,718 $ 88,229 $ 1,316,187

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 29, 2018
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the IXYS acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: 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 ) $ 778,788 The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the Monolith acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: Original investment $ 3,500 Cash, net of cash acquired 14,172 Non-cash, fair value of commitment to purchase non-controlling interest 9,000 Total purchase consideration $ 26,672 Allocation of consideration to assets acquired and liabilities assumed: Current assets, net $ 891 Property, plant, and equipment 789 Patented and unpatented technologies 6,720 Non-compete agreement 140 Goodwill 20,641 Current liabilities (639 ) Other non-current liabilities (1,870 ) $ 26,672 The following table summarizes the purchase price allocation of the fair value of assets acquired and liabilities assumed in the U.S. Sensor acquisition: (in thousands) Purchase Price Allocation Total purchase consideration: Cash $ 24,340 Allocation of consideration to assets acquired and liabilities assumed: Current assets, net $ 4,635 Patented and unpatented technologies 1,090 Trademarks and tradenames 200 Non-compete agreement 50 Customer relationships 2,830 Goodwill 16,075 Current liabilities (540 ) $ 24,340
Business Acquisition, Pro Forma Information	The pro forma amounts presented are not necessarily indicative of either the actual consolidated results had the IXYS acquisition occurred as of January 1, 2017 or of future consolidated operating results. Three Months Ended (in thousands, except per share amounts) September 29, 2018 September 30, Net sales $ 439,191 $ 405,573 Income before income taxes 71,737 50,883 Net income 56,060 40,004 Net income per share — basic 2.23 1.61 Net income per share — diluted $ 2.18 $ 1.59 Nine Months Ended (in thousands, except per share amounts) September 29, September 30, Net sales $ 1,332,900 $ 1,171,283 Income before income taxes 228,503 102,429 Net income 179,264 95,047 Net income — basic 7.17 3.84 Net income — diluted $ 7.16 $ 3.78
Business Acquisition, Pro Forma Information, Nonrecurring Adjustments	Pro forma results presented above primarily reflect the following adjustments: Three Months Ended Nine Months Ended (in thousands) September 29, September 30, September 29, September 30, Amortization (a) $ 3,104 $ (6,304 ) $ 8,289 $ (18,905 ) Depreciation — 139 — 417 Transaction costs (b) — — 9,976 (9,976 ) Amortization of inventory step-up (c) — — 36,927 (36,927 ) Stock compensation (d) 421 (426 ) 5,110 (6,206 ) Interest expense (e) — (2,582 ) — (7,746 ) Income tax impact of above items $ (1,011 ) $ 2,906 $ (14,290 ) $ 25,802 (a) The amortization adjustment for the nine months ended September 29, 2018 primarily reflects the reduction of amortization expense in the period related to the Order backlog intangible asset. The Order backlog has a useful life of twelve months and will be fully amortized in the fiscal 2017 pro forma results. The amortization adjustment for the three and nine months ended September 30, 2017 reflects incremental amortization resulting for the measurement of intangibles at their fair values. (b) The transaction cost adjustments reflect the reversal of certain bank and attorney fees from the nine months ended September 29, 2018 and recognition of those fees during the nine months ended September 30, 2017 . (c) The amortization of inventory step-up adjustment reflects the reversal of the amount recognized during the nine months ended September 29, 2018 and the recognition of the full amortization during the nine months ended September 30, 2017 . The inventory step-up was amortized over five months as the inventory was sold. (d) The stock compensation adjustment reflects the reversal of the portion of stock compensation for IXYS stock options that were converted to Littelfuse stock options and expensed immediately during the nine months ended September 29, 2018 . The adjustment for the nine months ended September 30, 2017 reflect the incremental stock compensation for the converted stock options. (e) The interest expense adjustment reflects incremental interest expense related to the financing of the transaction.

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 29, 2018
Inventory Disclosure [Abstract]
Schedule of Inventory	The components of inventories at September 29, 2018 and December 30, 2017 are as follows: (in thousands) September 29 December 30 Raw materials $ 69,575 $ 39,030 Work in process 83,115 27,454 Finished goods 94,565 74,305 Total $ 247,255 $ 140,789

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	9 Months Ended
	Sep. 29, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The amounts for goodwill and changes in the carrying value by segment for the nine months ended September 29, 2018 are as follows: (in thousands) Electronics Automotive Industrial Total As of December 30, 2017 $ 278,959 $ 135,829 $ 38,626 $ 453,414 Additions (a) 380,199 — — 380,199 Currency translation (1,090 ) (2,040 ) (129 ) (3,259 ) As of September 29, 2018 $ 658,068 $ 133,789 $ 38,497 $ 830,354 (a) The additions resulted from the acquisition of IXYS and an immaterial acquisition.
Schedule of Finite-Lived Intangible Assets	The components of other intangible assets at September 29, 2018 are as follows: (in thousands, except weighted average useful life) Weighted Average Useful Life (Years) Gross Carrying Value Accumulated Amortization Net Book Value Patents, licenses and software 10.5 $ 192,215 $ 71,451 $ 120,764 Distribution network 12.6 44,098 34,214 9,884 Customer relationships, trademarks, and tradenames 18 312,267 69,478 242,789 Order backlog 0.3 12,420 8,706 3,714 Total $ 561,000 $ 183,849 $ 377,151 During the nine months ended September 29, 2018 , the Company recorded additions to other intangible assets of $212.7 million , related to the IXYS acquisition, the components of which were as follows: (in thousands, except weighted average useful life) Weighted Average Useful Life (Years) Amount Patents, licenses and software 8 $ 51,500 Customer relationships, trademarks, and tradenames 17.2 148,800 Order backlog 1 12,420 Total $ 212,720
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	Estimated annual amortization expense related to intangible assets with definite lives as of September 29, 2018 is as follows: (in thousands) Amount 2018 $ 53,383 2019 40,454 2020 40,237 2021 38,403 2022 37,506 2023 and thereafter 206,586 Total $ 416,569

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 29, 2018
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments	The carrying amounts of debt at September 29, 2018 and December 30, 2017 are as follows: (in thousands) September 29, December 30, Term Loan $ 155,000 $ 122,500 Euro Senior Notes, Series A due 2023 136,819 139,623 Euro Senior Notes, Series B due 2028 111,092 113,369 U.S. Senior Notes, Series A due 2022 25,000 25,000 U.S. Senior Notes, Series B due 2027 100,000 100,000 U.S. Senior Notes, Series A due 2025 50,000 — U.S. Senior Notes, Series B due 2030 125,000 — Other 2,694 — Unamortized debt issuance costs (4,892 ) (4,881 ) Total debt 700,713 495,611 Less: Current maturities (10,076 ) (6,250 ) Total long-term debt $ 690,637 $ 489,361

Fair Value of Assets and Liab_2

Fair Value of Assets and Liabilities (Tables)	9 Months Ended
	Sep. 29, 2018
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring Basis	The following table presents assets measured at fair value by classification within the fair value hierarchy as of December 30, 2017 : Fair Value Measurements Using (in thousands) Quoted Prices in Significant Significant Total Investments in equity securities $ 10,993 $ — $ — $ 10,993 Mutual funds 7,962 — — 7,962 The following table presents assets measured at fair value by classification within the fair value hierarchy as of September 29, 2018 : Fair Value Measurements Using (in thousands) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Investments in equity securities $ 12,204 $ — $ — $ 12,204 Mutual funds 10,546 — — 10,546
Fair Value, by Balance Sheet Grouping	The carrying value and estimated fair values of the Company’s Euro Senior Notes, Series A and Series B and USD Senior Notes, Series A and Series B, as of September 29, 2018 and December 30, 2017 were as follows: September 29, 2018 December 30, 2017 (in thousands) Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Euro Senior Notes, Series A due 2023 $ 136,819 $ 135,392 $ 139,623 $ 138,294 Euro Senior Notes, Series B due 2028 111,092 108,511 113,369 111,579 USD Senior Notes, Series A due 2022 25,000 24,052 25,000 24,737 USD Senior Notes, Series B due 2027 100,000 94,474 100,000 99,992 USD Senior Notes, Series A due 2025 50,000 47,333 — — USD Senior Notes, Series B due 2030 125,000 115,231 — —

Benefit Plans (Tables)

Benefit Plans (Tables)	9 Months Ended
	Sep. 29, 2018
Retirement Benefits [Abstract]
Schedule of Net Benefit Costs	The components of net periodic benefit cost for the three and nine months ended September 29, 2018 and September 30, 2017 were as follows: For the Three Months Ended For the Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Components of net periodic benefit cost: Service cost $ 533 $ 408 $ 1,599 $ 1,224 Interest cost 501 360 1,503 1,080 Expected return on plan assets (540 ) (476 ) (1,620 ) (1,428 ) Amortization of prior service 74 84 222 252 Net periodic benefit cost $ 568 $ 376 $ 1,704 $ 1,128

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	9 Months Ended
	Sep. 29, 2018
Stockholders' Equity Note [Abstract]
Schedule of Stockholders Equity	The following table sets forth the changes in shareholders’ equity for the nine months ended September 29, 2018 : (in thousands) Littelfuse, Inc. Shareholders’ Equity Non- controlling Interest Total Balance at December 30, 2017 927,419 137 927,556 Net income 131,900 — 131,900 Other comprehensive loss (24,168 ) — (24,168 ) Stock-based compensation 23,153 — 23,153 Withheld shares on restricted share units for withholding taxes (7,252 ) — (7,252 ) Stock options exercised 25,171 — 25,171 Issuance of common stock (a) 472,301 — 472,301 Cash dividends paid (29,258 ) — (29,258 ) Non-controlling interest — (7 ) (7 ) Balance at September 29, 2018 1,519,266 130 1,519,396 (a) The issuance of common stock ( 2,092,491 shares) during the nine months ended September 29, 2018 relates to the acquisition of IXYS. See Note 2, Acquisitions for further discussion

Other Comprehensive Loss (Inc_2

Other Comprehensive Loss (Income) (Tables)	9 Months Ended
	Sep. 29, 2018
Equity [Abstract]
Schedule of Components of Comprehensive Income (Loss)	Change in other comprehensive (loss) income by component were as follows: (in thousands) Three Months Ended September 29, 2018 Three Months Ended September 30, 2017 Pre-tax Tax Net of Tax Pre-tax Tax Net of Tax Defined benefit pension plans adjustments $ (115 ) $ — $ (115 ) $ (149 ) $ 89 $ (60 ) Unrealized loss on investments — — — (1,710 ) — (1,710 ) Foreign currency translation adjustments (7,832 ) — (7,832 ) 1,531 — 1,531 Total change in other comprehensive (loss) income $ (7,947 ) $ — $ (7,947 ) $ (328 ) $ 89 $ (239 ) (in thousands) Nine Months Ended September 29, 2018 Nine Months Ended September 30, 2017 Pre-tax Tax Net of Tax Pre-tax Tax Net of Tax Defined benefit pension plans adjustments $ 630 $ (18 ) $ 648 $ (585 ) $ 144 $ (441 ) Unrealized loss on investments — — — (1,235 ) — (1,235 ) Foreign currency translation adjustments (24,816 ) — (24,816 ) 2,912 — 2,912 Total change in other comprehensive (loss) income $ (24,186 ) $ (18 ) $ (24,168 ) $ 1,092 $ 144 $ 1,236
Schedule of Accumulated Other Comprehensive Income (Loss)	The following table sets forth the changes in accumulated other comprehensive (loss) income by component for the nine months ended September 29, 2018 : (in thousands) Pension and postretirement liability and reclassification adjustments Unrealized gain (loss) on investments Foreign currency translation adjustment Accumulated other comprehensive income (loss) Balance at December 30, 2017 $ (10,836 ) $ 9,795 $ (62,627 ) $ (63,668 ) Cumulative effect adjustment (a) — (9,795 ) — (9,795 ) Activity in the period 648 — (24,816 ) (24,168 ) Balance at September 29, 2018 $ (10,188 ) $ — $ (87,443 ) $ (97,631 ) (a) The Company adopted ASU 2016-01 on December 31, 2017 on a modified retrospective basis, recognizing the cumulative effect as a $9.8 million increase to retained earnings. See Note 1, Summary of Significant Accounting Policies and Other Information , for further discussion.
Reclassification out of Accumulated Other Comprehensive Income	Amounts reclassified from accumulated other comprehensive (loss) income to earnings for the three and nine months ended September 29, 2018 and the three and nine months September 30, 2017 were as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Pension and Postemployment plans: Amortization of prior service $ 74 $ 84 $ 222 $ 252

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 29, 2018
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	The following table sets forth the computation of basic and diluted earnings per share: Three Months Ended Nine Months Ended (in thousands, except per share amounts) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Numerator: Net income as reported 53,546 42,808 131,900 130,338 Denominator: Weighted average shares outstanding Basic 25,109 22,713 24,817 22,678 Effect of dilutive securities 362 240 395 228 Diluted 25,471 22,953 25,212 22,906 Earnings Per Share: Basic earnings per share 2.13 1.88 5.31 5.75 Diluted earnings per share 2.10 1.87 5.23 5.69

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 29, 2018
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment	Segment information is summarized as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Net sales Electronics $ 296,472 $ 175,899 $ 860,240 $ 499,052 Automotive 114,416 113,797 367,718 338,094 Industrial 28,303 28,193 88,229 79,539 Total net sales $ 439,191 $ 317,889 $ 1,316,187 $ 916,685 Depreciation and amortization Electronics $ 15,898 $ 8,986 $ 45,227 $ 26,080 Automotive 5,891 5,622 17,830 16,572 Industrial 1,364 1,338 4,291 3,982 Other 3,105 — 8,712 — Total depreciation and amortization $ 26,258 $ 15,946 $ 76,060 $ 46,634 Operating income (loss) Electronics $ 72,464 $ 44,345 $ 193,739 $ 122,518 Automotive 10,863 16,821 44,965 47,599 Industrial 4,134 3,757 14,123 5,769 Other (a) (11,233 ) (6,314 ) (79,406 ) (8,155 ) Total operating income $ 76,228 $ 58,609 $ 173,421 $ 167,731 Interest expense 5,775 3,467 16,980 9,868 Foreign exchange loss (gain) 982 632 (6,372 ) (1,483 ) Other expense (income), net 1,259 (1,013 ) (2,362 ) (962 ) Income before income taxes $ 68,212 $ 55,523 $ 165,175 $ 160,308 (a) Included in “Other” Operating income (loss) for the 2018 third quarter is $11.2 million ( $79.4 million year-to-date) of charges primarily related to IXYS acquisition, which include $36.9 million year-to-date of purchase accounting inventory step-up charges previously recorded during the first and second quarters of 2018, $2.5 million ( $16.3 million year-to-date) in acquisition-related and integration costs primarily related to legal, accounting and other expenses, $3.1 million ( $8.7 million year-to-date) in backlog amortization costs, $4.6 million ( $9.4 million year-to-date) of severance and other restructuring charges, and $4.5 million year-to-date stock compensation expense recognized immediately upon close for converted IXYS options related to prior services periods. In addition, there were $1.0 million ( $3.6 million year-to-date) of severance, other restructuring charges and acquisition-related expenses for other contemplated acquisitions which included $1.1 million year-to-date associated with the exit of the Custom business in the second quarter.
Schedule of Disclosure on Geographic Areas, Long-Lived Assets in Individual Foreign Countries by Country	The Company’s long-lived assets by country, as of September 29, 2018 and December 30, 2017 , were as follows: (in thousands) September 29, December 30, Long-lived assets United States $ 66,098 $ 23,490 China (a) 89,856 86,866 Mexico 69,395 62,510 Germany 37,281 1,082 Philippines 31,589 31,129 Other countries 49,664 45,500 Total long-lived assets $ 343,883 $ 250,577 (a) Includes mainland China, Taiwan, and Hong Kong. The Company’s additions to long-lived assets by country were as follows: Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 Additions to long-lived assets United States $ 5,636 $ 2,752 China (a) 19,043 22,165 Mexico 14,089 15,041 Germany 5,917 67 Philippines 6,133 2,018 Other countries 5,128 6,427 Total additions to long-lived assets $ 55,946 $ 48,470 (a) Includes mainland China, Taiwan, and Hong Kong. The Company’s net sales by country are as follows: Three Months Ended Nine Months Ended (in thousands) September 29, 2018 September 30, 2017 September 29, 2018 September 30, 2017 Net sales United States $ 125,867 $ 103,233 $ 386,980 $ 290,538 China (a) 117,813 82,986 352,097 240,731 Other countries (b) 195,511 131,670 577,110 385,416 Total net sales $ 439,191 $ 317,889 $ 1,316,187 $ 916,685 (a) Includes mainland China, Taiwan, and Hong Kong. (b) Each country included in other countries are less than 10% of net sales.

Summary of Significant Accoun_4

Summary of Significant Accounting Policies and Other Information (Details Textual) - ASU 2016-01 $ in Millions	9 Months Ended
	Sep. 29, 2018USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	$ 9.8
Gain recognized on statement of income	$ 0.3

Summary of Significant Accoun_5

Summary of Significant Accounting Policies and Other Information - Revenue Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 439,191	$ 317,889	$ 1,316,187	$ 916,685
Electronics – Passive Products and Sensors
Disaggregation of Revenue [Line Items]
Net sales	124,174		366,990
Electronics – Semiconductor
Disaggregation of Revenue [Line Items]
Net sales	172,298		493,250
Passenger Car Products
Disaggregation of Revenue [Line Items]
Net sales	57,761		184,922
Automotive Sensors
Disaggregation of Revenue [Line Items]
Net sales	27,311		89,362
Commercial Vehicle Products
Disaggregation of Revenue [Line Items]
Net sales	29,344		93,434
Industrial Products
Disaggregation of Revenue [Line Items]
Net sales	28,303		88,229
Electronics Segment
Disaggregation of Revenue [Line Items]
Net sales	296,472	175,899	860,240	499,052
Electronics Segment | Electronics – Passive Products and Sensors
Disaggregation of Revenue [Line Items]
Net sales	124,174		366,990
Electronics Segment | Electronics – Semiconductor
Disaggregation of Revenue [Line Items]
Net sales	172,298		493,250
Electronics Segment | Passenger Car Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Electronics Segment | Automotive Sensors
Disaggregation of Revenue [Line Items]
Net sales	0		0
Electronics Segment | Commercial Vehicle Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Electronics Segment | Industrial Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Automotive Segment
Disaggregation of Revenue [Line Items]
Net sales	114,416	113,797	367,718	338,094
Automotive Segment | Electronics – Passive Products and Sensors
Disaggregation of Revenue [Line Items]
Net sales	0		0
Automotive Segment | Electronics – Semiconductor
Disaggregation of Revenue [Line Items]
Net sales	0		0
Automotive Segment | Passenger Car Products
Disaggregation of Revenue [Line Items]
Net sales	57,761		184,922
Automotive Segment | Automotive Sensors
Disaggregation of Revenue [Line Items]
Net sales	27,311		89,362
Automotive Segment | Commercial Vehicle Products
Disaggregation of Revenue [Line Items]
Net sales	29,344		93,434
Automotive Segment | Industrial Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment
Disaggregation of Revenue [Line Items]
Net sales	28,303	$ 28,193	88,229	$ 79,539
Industrial Segment | Electronics – Passive Products and Sensors
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment | Electronics – Semiconductor
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment | Passenger Car Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment | Automotive Sensors
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment | Commercial Vehicle Products
Disaggregation of Revenue [Line Items]
Net sales	0		0
Industrial Segment | Industrial Products
Disaggregation of Revenue [Line Items]
Net sales	$ 28,303		$ 88,229

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands	Oct. 05, 2018USD ($)	Jan. 17, 2018USD ($)customer$ / shares	Jul. 07, 2017USD ($)	Mar. 01, 2017USD ($)	Feb. 28, 2017USD ($)	Apr. 30, 2018USD ($)	Sep. 29, 2018USD ($)$ / shares	Jun. 30, 2018USD ($)	Sep. 29, 2018USD ($)$ / shares	Sep. 30, 2017USD ($)	Dec. 30, 2017$ / shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares							$ 0.01		$ 0.01		$ 0.01
Non-cash inventory charges									$ 36,927	$ 1,607
Cash, net of cash acquired									313,475	38,610
Non-cash, fair value of commitment to purchase non-controlling interest									$ 5,000	$ 9,000
Inventory amortization period									5 months
IXYS Corporation
Business Acquisition [Line Items]
Number of customers | customer		3,500
Common stock, par value (in dollars per share) | $ / shares		$ 0.01
Share price (in dollars per share) | $ / shares		$ 23
Share exchange ratio		0.1265
Share price (in dollars per share) | $ / shares		$ 207.5
Consideration excluding value of stock options converted		$ 814,800
Cash		380,600
Littelfuse stock		434,192
Value of options converted		41,700
Consideration transferred		856,500
Receivables		49,100
Revenue of acquiree since acquisition							$ 99,700	$ 286,200
Earnings (loss) of acquiree since acquisition							6,200	$ (25,500)
Step-up of inventory at acquisition		36,900
Cash, net of cash acquired		302,865
IXYS Corporation | Selling, General and Administrative Expenses
Business Acquisition [Line Items]
Allocated share based compensation		$ 4,500					$ 4,300		$ 11,800
Acquisition related costs									$ 11,000
U.S. Sensor
Business Acquisition [Line Items]
Cash			$ 24,340
Receivables			1,500
Inventory adjustment			$ 1,600
Monolith
Business Acquisition [Line Items]
Consideration transferred					$ 15,000	$ 5,000
Receivables					$ 700
Cost method investments												$ 3,500	$ 3,500
Ownership percentage by noncontrolling owners													12.00%
Voting interest acquired					62.00%	19.00%
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
Monolith | Stockholders of Monolith
Business Acquisition [Line Items]
Consideration transferred						$ 4,000
Monolith | Subsequent Event
Business Acquisition [Line Items]
Consideration transferred	$ 5,000
Voting interest acquired	19.00%

Acquisitions - Preliminary Pric

Acquisitions - Preliminary Price Allocation (Details) - USD ($) $ in Thousands	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

Acquisitions - Business Acquisi

Acquisitions - Business Acquisition Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Business Combinations [Abstract]
Net sales	$ 439,191	$ 405,573	$ 1,332,900	$ 1,171,283
Income before income taxes	71,737	50,883	228,503	102,429
Net income	$ 56,060	$ 40,004	$ 179,264	$ 95,047
Net income per share — basic (in dollars per share)	$ 2.23	$ 1.61	$ 7.17	$ 3.84
Net income per share — diluted (in dollars per share)	$ 2.18	$ 1.59	$ 7.16	$ 3.78

Acquisition - Pro Forma Informa

Acquisition - Pro Forma Information Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	$ 56,060	$ 40,004	$ 179,264	$ 95,047
Amortization
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	3,104	(6,304)	8,289	(18,905)
Depreciation
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	0	139	0	417
Transaction costs
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	0	0	9,976	(9,976)
Amortization of inventory step-up
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	0	0	36,927	(36,927)
Stock compensation
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	421	(426)	5,110	(6,206)
Interest expense
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	0	(2,582)	0	(7,746)
Income tax impact of above items
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net income	$ (1,011)	$ 2,906	$ (14,290)	$ 25,802

Inventories - Components of Inv

Inventories - Components of Inventories (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 69,575	$ 39,030
Work in process	83,115	27,454
Finished goods	94,565	74,305
Total	$ 247,255	$ 140,789

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Amounts for Goodwill and Changes in Carrying Value by Operating Segment (Details) $ in Thousands	9 Months Ended
	Sep. 29, 2018USD ($)
Goodwill [Roll Forward]
Balance	$ 453,414
Additions	380,199
Currency translation	(3,259)
Balance	830,354
Electronics
Goodwill [Roll Forward]
Balance	278,959
Additions	380,199
Currency translation	(1,090)
Balance	658,068
Automotive
Goodwill [Roll Forward]
Balance	135,829
Additions	0
Currency translation	(2,040)
Balance	133,789
Industrial
Goodwill [Roll Forward]
Balance	38,626
Additions	0
Currency translation	(129)
Balance	$ 38,497

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Amount			$ 212,720
Amortization of intangibles	$ 13,130	$ 6,292	38,501	$ 18,407
IXYS Corporation
Finite-Lived Intangible Assets [Line Items]
Amount			$ 212,700

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Details of Other Intangible Assets and Related Future Amortization Expense (Details) $ in Thousands	9 Months Ended
	Sep. 29, 2018USD ($)
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 561,000
Accumulated Amortization	183,849
Net Book Value	377,151
Amount	$ 212,720
Patents, licenses and software
Finite-Lived Intangible Assets [Line Items]
Weighted Average Useful Life (Years)	10 years 6 months
Gross Carrying Value	$ 192,215
Accumulated Amortization	71,451
Net Book Value	$ 120,764
Weighted Average Useful Life (Years)	8 years
Amount	$ 51,500
Distribution network
Finite-Lived Intangible Assets [Line Items]
Weighted Average Useful Life (Years)	12 years 7 months 6 days
Gross Carrying Value	$ 44,098
Accumulated Amortization	34,214
Net Book Value	$ 9,884
Customer relationships, trademarks, and tradenames
Finite-Lived Intangible Assets [Line Items]
Weighted Average Useful Life (Years)	18 years
Gross Carrying Value	$ 312,267
Accumulated Amortization	69,478
Net Book Value	$ 242,789
Weighted Average Useful Life (Years)	17 years 2 months 12 days
Amount	$ 148,800
Order backlog
Finite-Lived Intangible Assets [Line Items]
Weighted Average Useful Life (Years)	3 months 18 days
Gross Carrying Value	$ 12,420
Accumulated Amortization	8,706
Net Book Value	$ 3,714
Weighted Average Useful Life (Years)	1 year
Amount	$ 12,420

Goodwill and Other Intangible_6

Goodwill and Other Intangible Assets - Estimated Amortization Expense Related to Intangible Assets with Definite Lives (Details) $ in Thousands	Sep. 29, 2018USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,018	$ 53,383
2,019	40,454
2,020	40,237
2,021	38,403
2,022	37,506
2023 and thereafter	206,586
Total	$ 416,569

Debt - Carrying Amounts of Long

Debt - Carrying Amounts of Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (4,892)	$ (4,881)
Total debt	700,713	495,611
Less: Current maturities	(10,076)	(6,250)
Total long-term debt	690,637	489,361
Term Loan
Debt Instrument [Line Items]
Long-term debt, gross	155,000	122,500
Euro Senior Notes, Series A due 2023
Debt Instrument [Line Items]
Long-term debt, gross	136,819	139,623
Euro Senior Notes, Series B due 2028
Debt Instrument [Line Items]
Long-term debt, gross	111,092	113,369
U.S. Senior Notes, Series A due 2022
Debt Instrument [Line Items]
Long-term debt, gross	25,000	25,000
U.S. Senior Notes, Series B due 2027
Debt Instrument [Line Items]
Long-term debt, gross	100,000	100,000
U.S. Senior Notes, Series A due 2025
Debt Instrument [Line Items]
Long-term debt, gross	50,000	0
U.S. Senior Notes, Series B due 2030
Debt Instrument [Line Items]
Long-term debt, gross	125,000	0
Other
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,694	$ 0

Debt - Narrative (Details)

Debt - Narrative (Details)	Jan. 16, 2018USD ($)	Oct. 13, 2017USD ($)advance	Mar. 04, 2016USD ($)	Sep. 29, 2018USD ($)	Dec. 30, 2017	Nov. 15, 2017USD ($)series	Oct. 12, 2017USD ($)	Feb. 15, 2017USD ($)	Dec. 08, 2016EUR (€)series	Dec. 08, 2016USD ($)series
Credit Agreement
Debt Instrument [Line Items]
Debt term			5 years
Maximum borrowing capacity			$ 700,000,000
Additional uncommitted borrowing capacity		$ 300,000,000	150,000,000
Additional uncommitted borrowing capacity, minimum increments			$ 25,000,000
Number of advances | advance		2
Effective interest rate				3.49%
Letter of credit oustanding				$ 100,000
Credit Agreement | Minimum
Debt Instrument [Line Items]
Commitment fee			0.15%
Credit Agreement | Maximum
Debt Instrument [Line Items]
Commitment fee			0.25%
Credit Agreement | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			1.00%
Credit Agreement | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			2.00%
Credit Agreement | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			0.00%
Credit Agreement | Base Rate | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			1.00%
Credit Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 700,000,000	$ 575,000,000				$ 575,000,000
Remaining borrowing capacity				699,900,000
Credit Agreement | Term Loan Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity, credit facility		200,000,000	$ 125,000,000
Proceeds from issuance of debt	$ 75,000,000	$ 125,000,000
Quarterly payments, percentage of loan					1.25%
Periodic payment	2,500,000
Repayments of debt				$ 35,000,000
Euro Senior Notes, Series A and B
Debt Instrument [Line Items]
Face amount of debt | €									€ 212,000,000
Number of series | series						2			2	2
Euro Senior Notes, Series A due 2023
Debt Instrument [Line Items]
Face amount of debt | €									€ 117,000,000
Stated interest rate									1.14%	1.14%
Euro Senior Notes, Series B due 2028
Debt Instrument [Line Items]
Face amount of debt | €									€ 95,000,000
Stated interest rate									1.83%	1.83%
U.S. Senior Notes, Series A and B
Debt Instrument [Line Items]
Face amount of debt										$ 125,000,000
U.S. Senior Notes, Series A
Debt Instrument [Line Items]
Face amount of debt								$ 25,000,000
Stated interest rate								3.03%
U.S. Senior Notes, Series B
Debt Instrument [Line Items]
Face amount of debt								$ 100,000,000
Stated interest rate								3.74%
US Senior Notes A and B Due 2025 and 2030
Debt Instrument [Line Items]
Face amount of debt						$ 175,000,000
U.S. Senior Notes, Series A due 2025
Debt Instrument [Line Items]
Face amount of debt	$ 50,000,000
Stated interest rate	3.48%
U.S. Senior Notes, Series B due 2030
Debt Instrument [Line Items]
Face amount of debt	$ 125,000,000
Stated interest rate	3.78%

Fair Value of Assets and Liab_3

Fair Value of Assets and Liabilities - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	$ 12,204	$ 10,993
Mutual funds	10,546	7,962
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	12,204	10,993
Mutual funds	10,546	7,962
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	0	0
Mutual funds	0	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	0	0
Mutual funds	$ 0	$ 0

Fair Value of Assets and Liab_4

Fair Value of Assets and Liabilities - Carrying Value and Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 30, 2017
Carrying Value | Euro Senior Notes, Series A due 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	$ 136,819	$ 139,623
Carrying Value | Euro Senior Notes, Series B due 2028
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	111,092	113,369
Carrying Value | U.S. Senior Notes, Series A due 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	25,000	25,000
Carrying Value | U.S. Senior Notes, Series B due 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	100,000	100,000
Carrying Value | U.S. Senior Notes, Series A due 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	50,000	0
Carrying Value | U.S. Senior Notes, Series B due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	125,000	0
Estimated Fair Value | Euro Senior Notes, Series A due 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	135,392	138,294
Estimated Fair Value | Euro Senior Notes, Series B due 2028
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	108,511	111,579
Estimated Fair Value | U.S. Senior Notes, Series A due 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	24,052	24,737
Estimated Fair Value | U.S. Senior Notes, Series B due 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	94,474	99,992
Estimated Fair Value | U.S. Senior Notes, Series A due 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	47,333	0
Estimated Fair Value | U.S. Senior Notes, Series B due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	$ 115,231	$ 0

Benefit Plans - Narrative (Deta

Benefit Plans - Narrative (Details) $ in Millions	Sep. 29, 2018USD ($)
Retirement Benefits [Abstract]
Contributions expected	$ 2.3

Benefit Plans - Benefit Plan Ex

Benefit Plans - Benefit Plan Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 533	$ 408	$ 1,599	$ 1,224
Interest cost	501	360	1,503	1,080
Expected return on plan assets	(540)	(476)	(1,620)	(1,428)
Amortization of prior service	74	84	222	252
Net periodic benefit cost	$ 568	$ 376	$ 1,704	$ 1,128

Shareholders' Equity - Narrativ

Shareholders' Equity - Narrative (Details) - USD ($) $ in Millions	Oct. 29, 2018	Sep. 29, 2018
Subsequent Event
Class of Stock [Line Items]
Stock repurchased (in shares)	200,000
Stock repurchased	$ 35.9
IXYS Corporation
Class of Stock [Line Items]
Stock issued for acquisition (in shares)		2,092,491

Shareholders' Equity - Changes

Shareholders' Equity - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			$ 927,556
Net income	$ 53,546	$ 42,808	131,900	$ 130,338
Other comprehensive loss	(7,947)	$ (239)	(24,168)	$ 1,236
Stock-based compensation			23,153
Withheld shares on restricted share units for withholding taxes			(7,252)
Stock options exercised			25,171
Issuance of common stock			472,301
Cash dividends paid			(29,258)
Non-controlling interest			(7)
Balance	1,519,396		1,519,396
Littelfuse, Inc. Shareholders’ Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			927,419
Net income			131,900
Other comprehensive loss			(24,168)
Stock-based compensation			23,153
Withheld shares on restricted share units for withholding taxes			(7,252)
Stock options exercised			25,171
Issuance of common stock			472,301
Cash dividends paid			(29,258)
Balance	1,519,266		1,519,266
Non- controlling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			137
Non-controlling interest			(7)
Balance	$ 130		$ 130

Other Comprehensive Loss (Inc_3

Other Comprehensive Loss (Income) - Schedule of Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Other Comprehensive Income (Loss) [Line Items]
Pre-tax	$ (7,947)	$ (328)	$ (24,186)	$ 1,092
Tax	0	89	(18)	144
Total change in other comprehensive (loss) income	(7,947)	(239)	(24,168)	1,236
Pension and postretirement liability and reclassification adjustments
Other Comprehensive Income (Loss) [Line Items]
Pre-tax	(115)	(149)	630	(585)
Tax	0	89	(18)	144
Total change in other comprehensive (loss) income	(115)	(60)	648	(441)
Unrealized gain (loss) on investments
Other Comprehensive Income (Loss) [Line Items]
Pre-tax	0	(1,710)	0	(1,235)
Tax	0	0	0	0
Total change in other comprehensive (loss) income	0	(1,710)	0	(1,235)
Foreign currency translation adjustment
Other Comprehensive Income (Loss) [Line Items]
Pre-tax	(7,832)	1,531	(24,816)	2,912
Tax	0	0	0	0
Total change in other comprehensive (loss) income	$ (7,832)	$ 1,531	$ (24,816)	$ 2,912

Other Comprehensive Loss (Inc_4

Other Comprehensive Loss (Income) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance			$ 927,556
Activity in the period	$ (7,947)	$ (239)	(24,168)	$ 1,236
Balance	1,519,396		1,519,396
ASU 2016-01
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Cumulative effect adjustment			9,800
Pension and postretirement liability and reclassification adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance			(10,836)
Cumulative effect adjustment			0
Activity in the period	(115)	(60)	648	(441)
Balance	(10,188)		(10,188)
Unrealized gain (loss) on investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance			9,795
Cumulative effect adjustment			(9,795)
Activity in the period	0	(1,710)	0	(1,235)
Balance	0		0
Foreign currency translation adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance			(62,627)
Cumulative effect adjustment			0
Activity in the period	(7,832)	$ 1,531	(24,816)	$ 2,912
Balance	(87,443)		(87,443)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance			(63,668)
Cumulative effect adjustment			(9,795)
Activity in the period			(24,168)
Balance	$ (97,631)		$ (97,631)

Other Comprehensive Loss (Inc_5

Other Comprehensive Loss (Income) - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service	$ 259,597	$ 184,238	$ 817,983	$ 536,776
Amortization of prior service | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service	$ 74	$ 84	$ 222	$ 252

Income Taxes (Details Textual)

Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 29, 2018	Dec. 30, 2017	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017	Dec. 30, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective tax rate	21.50%		22.90%	20.10%	18.70%
Federal tax rate						35.00%
Impact of tax act		$ 47
Toll charge		49
Other than toll charge		$ 2
Undistributed foreign earnings							$ 12
Foreign Tax Authority
Income Tax Contingency [Line Items]
Undistributed foreign earnings							11.8
Domestic Tax Authority
Income Tax Contingency [Line Items]
Undistributed foreign earnings							$ 0.2
Other Long-term Liabilities
Income Tax Contingency [Line Items]
Toll charge	$ 29.7
IXYS Corporation
Income Tax Contingency [Line Items]
Toll charge	$ (2)			$ 8

Earnings Per Share - Narrative

Earnings Per Share - Narrative (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded (in shares)	38,082	30,579	39,446	35,858

Earnings Per Share - Computatio

Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Numerator:
Net income as reported	$ 53,546	$ 42,808	$ 131,900	$ 130,338
Weighted average shares outstanding
Basic (in shares)	25,109	22,713	24,817	22,678
Effect of dilutive securities (in shares)	362	240	395	228
Diluted (in shares)	25,471	22,953	25,212	22,906
Earnings Per Share:
Basic earnings per share (in dollars per share)	$ 2.13	$ 1.88	$ 5.31	$ 5.75
Diluted earnings per share (in dollars per share)	$ 2.10	$ 1.87	$ 5.23	$ 5.69

Related Party Transactions (Det

Related Party Transactions (Details Textual) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 29, 2018USD ($)	Sep. 29, 2018USD ($)
Powersem
Related Party Transaction [Line Items]
Revenue from related parties	$ 0.2	$ 0.6
Purchase from related party	1.3	3.4
Receivable from related parties	0.1	0.1
Payables to related parties	0.2	0.2
EB Tech
Related Party Transaction [Line Items]
Payables to related parties	0.1	0.1
Amount of transaction	0.1	0.3
ATEC
Related Party Transaction [Line Items]
Payables to related parties	0.6	0.6
Amount of transaction	$ 2.5	$ 7.7
Powersem
Related Party Transaction [Line Items]
Ownership percentage	45.00%	45.00%
EB Tech
Related Party Transaction [Line Items]
Ownership percentage	20.00%	20.00%
ATEC
Related Party Transaction [Line Items]
Ownership percentage	24.00%	24.00%

Segment Information - Segment I

Segment Information - Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 439,191	$ 317,889	$ 1,316,187	$ 916,685
Depreciation and amortization	26,258	15,946	76,060	46,634
Operating income (loss)	76,228	58,609	173,421	167,731
Interest expense	5,775	3,467	16,980	9,868
Foreign exchange loss (gain)	982	632	(6,372)	(1,483)
Other expense (income), net	1,259	(1,013)	(2,362)	(962)
Income before income taxes	68,212	55,523	165,175	160,308
Electronics
Segment Reporting Information [Line Items]
Net sales	296,472	175,899	860,240	499,052
Depreciation and amortization	15,898	8,986	45,227	26,080
Operating income (loss)	72,464	44,345	193,739	122,518
Automotive
Segment Reporting Information [Line Items]
Net sales	114,416	113,797	367,718	338,094
Depreciation and amortization	5,891	5,622	17,830	16,572
Operating income (loss)	10,863	16,821	44,965	47,599
Industrial
Segment Reporting Information [Line Items]
Net sales	28,303	28,193	88,229	79,539
Depreciation and amortization	1,364	1,338	4,291	3,982
Operating income (loss)	4,134	3,757	14,123	5,769
Other
Segment Reporting Information [Line Items]
Depreciation and amortization	3,105	0	8,712	0
Operating income (loss)	$ (11,233)	$ (6,314)	$ (79,406)	$ (8,155)

Segment Information - Narrative

Segment Information - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 29, 2018USD ($)segment	Sep. 30, 2017USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Impairment charges			$ 1,125	$ 0
Net sales | Geographic Concentration Risk | Other countries
Segment Reporting Information [Line Items]
Concentration risk (less than)			10.00%
Other | Operating Income (Loss)
Segment Reporting Information [Line Items]
Acquisition related costs	$ 11,200	$ 6,300	$ 79,400	8,200
Inventory adjustment		1,600	36,900	1,600
Legal, accounting and other expenses	2,500	4,800	16,300	6,600
Backlog amortization costs	3,100		8,700
Share based compensation			4,500
Restructuring charges	4,600	$ 1,500	9,400	$ 1,500
Impairment charges	$ 1,000		3,600
Other asset impairment charges			$ 1,100

Segment Information - Revenues

Segment Information - Revenues and Long-lived Assets by Geographical Area (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017	Dec. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 439,191	$ 317,889	$ 1,316,187	$ 916,685
Long-lived assets	343,883		343,883		$ 250,577
Additions to long-lived assets			55,946	48,470
United States
Segment Reporting Information [Line Items]
Net sales	125,867	103,233	386,980	290,538
Long-lived assets	66,098		66,098		23,490
Additions to long-lived assets			5,636	2,752
China
Segment Reporting Information [Line Items]
Net sales	117,813	82,986	352,097	240,731
Long-lived assets	89,856		89,856		86,866
Additions to long-lived assets			19,043	22,165
Mexico
Segment Reporting Information [Line Items]
Long-lived assets	69,395		69,395		62,510
Additions to long-lived assets			14,089	15,041
Germany
Segment Reporting Information [Line Items]
Long-lived assets	37,281		37,281		1,082
Additions to long-lived assets			5,917	67
Philippines
Segment Reporting Information [Line Items]
Long-lived assets	31,589		31,589		31,129
Additions to long-lived assets			6,133	2,018
Other countries
Segment Reporting Information [Line Items]
Net sales	195,511	$ 131,670	577,110	385,416
Long-lived assets	$ 49,664		49,664		$ 45,500
Additions to long-lived assets			$ 5,128	$ 6,427