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Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
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Jan. 17, 2018 USD ($) customer $ / shares | Dec. 26, 2020 USD ($) $ / shares | Sep. 26, 2020 USD ($) | Jun. 27, 2020 USD ($) | Mar. 28, 2020 USD ($) | Dec. 28, 2019 USD ($) $ / shares | Sep. 28, 2019 USD ($) | Jun. 29, 2019 USD ($) | Mar. 30, 2019 USD ($) | Dec. 29, 2018 USD ($) | Sep. 29, 2018 USD ($) | Jun. 30, 2018 USD ($) | Dec. 26, 2020 USD ($) $ / shares | Dec. 28, 2019 USD ($) $ / shares | Dec. 29, 2018 USD ($) | Dec. 30, 2017
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares | $ 0.01 | $ 0.01 |  |  |  | $ 0.01 |  |  |  |  |  |  | $ 0.01 | $ 0.01 |  |
Share price (in dollars per share) | $ / shares | 207.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  |  |  |  |  |  |  |  |  | $ 3,200 |  |  |  |  |  |
Allocated share based compensation |  |  |  |  |  |  |  |  |  |  |  |  | $ 19,100 | $ 19,900 | $ 28,200 |
Stock-based compensation |  |  |  |  |  |  |  |  |  |  |  |  | 18,129 | 19,046 | $ 27,431 |
Acquisition related costs |  | $ 700 | $ 300 | $ 100 | $ 1,200 | $ 1,900 | $ 3,200 | $ 1,500 | $ 2,400 |  |  |  |  |  |  |
Inventory amortization period (in years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 months |
Order backlog |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite lived intangible asset weighted average useful life (years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 months
Selling, General and Administrative Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 800 | $ 2,700 |  |
IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share price (in dollars per share) | $ / shares | $ 23.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share exchange ratio | 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in certain investments |  |  |  |  |  |  |  |  |  | 2,600 |  |  |  |  |  |
Revenue of acquiree since acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 378,200 |
Loss from acquiree since acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22,200 |
Stock-based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,500 |
Inventory adjustment |  |  |  |  |  |  |  |  |  |  |  | $ 36,900 |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,000 |
IXYS Corporation | Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition related costs |  |  |  |  |  |  |  |  |  | (1,000) |  |  |  |  |  |
IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of customers | customer | 3,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Toll Charge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge |  |  |  |  |  |  |  |  |  | 2,300 |  |  |  |  |  |
Toll Charge | IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
TCJA one-time charge | $ 7,000 |  |  |  |  |  |  |  |  | $ 7,000 | $ (2,000) |  |  |  |  |
Options | IXYS Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share based compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,900 |

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