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Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 05, 2018 USD ($) | Jan. 17, 2018 USD ($) customer $ / shares | Jul. 07, 2017 USD ($) | Mar. 01, 2017 USD ($) | Feb. 28, 2017 USD ($) | Apr. 30, 2018 USD ($) | Dec. 28, 2019 USD ($) $ / shares | Sep. 28, 2019 USD ($) | Jun. 29, 2019 USD ($) | Mar. 30, 2019 USD ($) | Dec. 29, 2018 USD ($) $ / shares | Sep. 29, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 28, 2019 USD ($) $ / shares | Dec. 29, 2018 USD ($) $ / shares | Jan. 17, 2018 USD ($) $ / shares | Dec. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 0.01 |  |  |  | $ 0.01 |  |  |  | $ 0.01 | $ 0.01 |  |  |  |
Share price (in dollars per share) | $ / shares |  | $ 207.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 207.5 |  |  |
TCJA one-time charge |  |  |  |  |  |  |  |  |  |  | $ 3,200 |  |  |  |  |  |  |  |  |
Allocated share based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 19,900 | $ 28,200 |  | $ 17,300 |  |
Stock-based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,046 | 27,431 |  | 16,315 |  |
Acquisition related costs |  |  |  |  |  |  | $ 1,900 | $ 3,200 | $ 1,500 | $ 2,400 | 3,200 | $ 2,900 | $ 2,400 | $ 11,700 |  |  |  |  |  |
Cost method investments |  |  |  |  |  |  | $ 400 |  |  |  | 7,900 |  |  |  | 400 | 7,900 |  |  |  |
Cash, net of cash acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 775 | 318,474 |  | 38,512 |  |
Inventory amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 months |  |  |  |  |
Order backlog |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average useful life (years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 months |  |  |  |  |
Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,700 |  |  |  |  |
IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ / shares |  | $ 23.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 23.00 |  |  |
Share exchange ratio |  | 0.1265 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.1265 |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 49,100 |  |  |
Reduction in certain investments |  |  |  |  |  |  |  |  |  |  | 2,600 |  |  |  |  |  |  |  |  |
Revenue of acquiree since acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 378,200 |  |  |  |  |
Earnings (loss) of acquiree since acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22,200 |  |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,200 | 11,000 |  |  |  |
Cash, net of cash acquired |  | 302,865 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total purchase consideration |  | $ 778,788 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IXYS Corporation | Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,500 |  |  |  |  |  |
U.S. Sensor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  | $ 24,340 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivables |  |  | 1,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Inventory adjustment |  |  | $ 1,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Monolith |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Consideration transferred | $ 5,000 |  |  |  | $ 15,000 | $ 5,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivables |  |  |  | $ 700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost method investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,500 | $ 3,500
Ownership percentage by noncontrolling owners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12.00%
Voting interest acquired | 19.00% |  |  |  | 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other | Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,000 |  |
Other | Operating Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Inventory adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 36,900 |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,900 | $ 88,700 |  |  |  |
IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of customers | customer |  | 3,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.01 |  |  |
Toll Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  |  |  |  |  |  |  |  |  |  | 2,300 |  |  |  |  |  |  |  |  |
Toll Charge | IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  | $ 7,000 |  |  |  |  |  |  |  |  | $ (1,000) | $ (2,000) |  |  |  |  | $ 7,000 |  |  |
Options | IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,900 |  |  |  |  |

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