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Common Stock and Stock-Based Compensation - Narrative (Detail) $ / shares in Units, $ in Thousands |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
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Nov. 02, 2020 USD ($) $ / shares shares | Oct. 02, 2020 USD ($) shares | Nov. 22, 2019 USD ($) | Oct. 31, 2017 USD ($) Stock $ / shares shares | Dec. 25, 2020 USD ($) $ / shares shares | Dec. 27, 2019 USD ($) shares | Dec. 25, 2020 USD ($) $ / shares shares | Dec. 27, 2019 USD ($) $ / shares shares | Mar. 27, 2020 USD ($) $ / shares shares | Mar. 29, 2019 USD ($) $ / shares shares | Sep. 25, 2020 shares | Sep. 27, 2019 shares | Mar. 29, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of classes of stock | Stock |  |  |  | 2 |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  | 1,000,000,000 |  | 1,000,000,000 |  | 1,000,000,000 |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  | $ 0.01 |  | $ 0.01 |  | $ 0.01 |  |  |  |
Number of awards subject to restriction |  |  |  |  | 377,767 |  | 125,922 |  |  |  |  |  |
Stock repurchased during period, aggregate purchase price | $ |  |  | $ 40,000 |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  | 189,431,726 |  | 189,431,726 |  | 0 |  |  |  |
Conversion of stock shares issued |  |  |  |  |  |  | 164,431,726 |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  | $ 45,876 | $ 303 | $ 46,901 | $ 1,051 | $ 1,435 | $ 1,441 |  |  |
Director | Non-employee |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
RSUs issued | 54,644 |  |  |  |  |  |  |  |  |  |  |  |
LTIP | RSU Conversion Program |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation | $ | $ 607 |  |  |  |  |  | 607 |  |  |  |  |  |
TRIP | RSU Conversion Program |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation | $ | $ 421 |  |  |  |  |  | $ 421 |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) |  |  |  |  | 25,000,000 |  | 25,000,000 |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  | 189,431,726 |  | 189,431,726 |  |  |  |  |  |
Shares Of Unvested Restricted Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Conversion of stock shares issued |  |  |  |  |  |  | 459,749 |  |  |  |  |  |
RSU |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period (in shares) |  |  |  |  |  |  | 1,426,944 |  |  |  |  |  |
Stock-based compensation expense, not yet recorded | $ |  |  |  |  |  |  | $ 17,496 |  |  |  |  |  |
Weighted-average period |  |  |  |  |  |  | 1 year 8 months 26 days |  | 0 years |  |  |  |
Grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 14.04 |  |  |  |  |  |
Weighted-average grant-date fair value | $ / shares |  |  |  |  | $ 14.04 |  | $ 14.04 |  |  |  |  |  |
Shares vested |  |  |  |  |  |  | 376 |  |  |  |  |  |
RSU | RSU Conversion Program |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Grant date fair value (in dollars per share) | $ / shares | $ 14.00 |  |  |  |  |  | $ 14 |  |  |  |  |  |
RSU | LTIP | RSU Conversion Program |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period (in shares) | 602,490 |  |  |  |  |  | 602,490 |  |  |  |  |  |
RSU | TRIP | RSU Conversion Program |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period (in shares) | 348,911 |  |  |  |  |  | 348,911 |  |  |  |  |  |
Target Performance Shares | Board of Directors Chairman |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation arrangement, Terms of award |  |  |  |  |  |  | Depending on the results achieved during the three-year performance period, the actual number of shares that a grant recipient may receive at the end of the period ranges from-% to 200% of the Target Shares granted. |  |  |  |  |  |
PSU |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Grants in period (in shares) |  |  |  |  |  |  | 650,302 |  |  |  |  |  |
Grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 15.05 |  |  |  |  |  |
Aggregate intrinsic value | $ |  |  |  |  | $ 16,121 |  | $ 16,121 |  |  |  |  |  |
Total compensation cost of non-vested awards | $ |  |  |  |  | $ 9,320 |  | $ 9,320 |  |  |  |  |  |
Weighted-average remaining contractual life |  |  |  |  |  |  | 2 years 10 months 24 days |  |  |  |  |  |
Number of shares vested |  |  |  |  | 0 |  | 0 |  |  |  |  |  |
PSU | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Vestinh rights percentage |  |  |  |  |  |  | 100.00% |  |  |  |  |  |
PSU | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Vestinh rights percentage |  |  |  |  |  |  | 200.00% |  |  |  |  |  |
IPO |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | 28,750,000 |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds | $ | $ 321,425 |  |  |  |  |  |  |  |  |  |  |  |
Estimated offering costs | $ | $ 8,450 |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding, weighted average price per share (in dollars per share) | $ / shares | $ 14.00 |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discounts | $ | $ 20,125 |  |  |  |  |  |  |  |  |  |  |  |
Shares issued upon conversion, including shares withheld for tax withholding obligation (in shares) | 166,500,000 |  |  |  |  |  |  |  |  |  |  |  |
IPO - Selling Shareholders |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | 3,750,000 |  |  |  |  |  |  |  |  |  |  |  |
Allegro Micro Systems Inc | IPO |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Common Class A |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  | 12,500,000 | 0 |  | 0 |  | 12,500,000 | 12,500,000 |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  | $ 0.01 | $ 0.01 |  | $ 0.01 |  | $ 0.01 | $ 0.01 |  |  |
Issuance of common shares (in shares) |  |  |  | 6,720,000 |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) |  |  |  | 2,880,000 |  |  |  |  |  |  |  |  |
Net proceeds | $ |  |  |  | $ 291 |  |  |  |  |  |  |  |  |
Estimated offering costs | $ |  |  |  | $ 9,260 |  |  |  |  |  |  |  |  |
Priority dividend, percentage |  |  |  | 8.00% |  |  |  |  |  |  |  |  |
Grants in period (in shares) |  |  |  | 400,000 |  |  |  |  |  |  |  |  |
Vesting period | 60 months |  |  |  |  |  |  |  | 60 months |  |  |  |
Shares returned for tax payments made on behalf of holders of common stock (in shares) | 2,066,508 |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  | 0 | 10,000,000 | 0 | 10,000,000 | 10,000,000 | 10,000,000 | 10,000,000 | 10,000,000 | 10,000,000
Conversion of stock shares issued |  |  |  |  |  |  | 156,155,403 |  |  |  |  |  |
Stock-based compensation | $ | $ 40,440 |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average grant-date fair value | $ / shares |  |  |  |  |  |  |  |  | $ 101.10 |  |  |  |
Common Class A | If Initial Public Offering or Change in Control Occurs |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated vesting cost | $ | $ 40,440 |  |  |  |  |  |  |  | $ 40,440 |  |  |  |
Accelerated vesting, number (in shares) | 400,000 |  |  |  |  |  |  |  | 400,000 |  |  |  |
Accelerated vesting, share price (in dollars per share) | $ / shares | $ 101.10 |  |  |  |  |  |  |  | $ 101.10 |  |  |  |
Common Class A | IPO |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Conversion rate | 15.822 |  |  |  |  |  |  |  |  |  |  |  |
Shares returned for tax payments made on behalf of holders of common stock (in shares) | 2,066,508 |  |  |  |  |  |  |  |  |  |  |  |
Common Class A | One Equity Partners [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest ownership percentage |  |  |  | 28.80% |  |  |  |  |  |  |  |  |
Common Class L |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  | 1,000,000 | 0 |  | 0 |  | 1,000,000 | 1,000,000 |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  | $ 0.01 | $ 0.01 |  | $ 0.01 |  | $ 0.01 | $ 0.01 |  |  |
Issuance of common shares (in shares) |  |  |  |  |  |  |  |  | 14,850 | 10,220 |  |  |
Grants in period (in shares) |  |  |  | 597,400 |  |  |  |  | 30,300 |  |  |  |
Vesting period |  |  |  | 4 years |  |  |  |  |  |  |  |  |
Accelerated vesting, percentage | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense, not yet recorded | $ |  |  |  |  |  |  |  |  | $ 2,970 |  |  |  |
Weighted-average period |  |  |  |  |  |  |  |  | 1 year 7 months 13 days |  |  |  |
Number of awards subject to restriction |  |  |  |  |  |  |  |  | 26,220 |  |  |  |
Stock repurchased during period (in shares) |  | 1,997 |  |  |  |  |  |  |  |  |  |  |
Stock repurchased during period, aggregate purchase price | $ |  | $ 408 |  |  |  |  |  |  |  |  |  |  |
Outstanding loan amounts extinguished | $ |  | $ 753 |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  | 0 | 607,620 | 0 | 607,620 | 622,470 | 607,620 | 638,298 | 607,620 | 597,400
Conversion of stock shares issued |  |  |  |  |  |  | 8,276,323 |  |  |  |  |  |
Stock-based compensation | $ | $ 1,610 |  |  |  | $ 144 |  | $ 1,169 |  |  |  |  |  |
Common Class L | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) |  |  |  |  |  |  | 17,203 | 30,300 |  |  |  |  |
Common stock, shares outstanding, weighted average price per share (in dollars per share) | $ / shares |  |  |  |  | $ 11.99 |  | $ 11.99 |  |  |  |  |  |
Shares issued during period, weighted average price per share (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 33.83 | $ 26.93 |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  | 638,298 |  | 638,298 |  |  |  |  |  |
Common Class L | Shares Of Unvested Restricted Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding, weighted average price per share (in dollars per share) | $ / shares |  |  |  |  | $ 14.00 |  | $ 14.00 |  |  |  |  |  |
Conversion of stock shares issued |  |  |  |  |  |  | 459,749 |  |  |  |  |  |
Common Class L | Shares Of Unvested Restricted Common Stock [Member] | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Conversion of stock shares issued |  |  |  |  |  |  | 7,816,574 |  |  |  |  |  |
Common Class L | Unvested Restricted Stock | Shares Of Unvested Restricted Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Conversion of stock shares issued |  |  |  |  |  |  | 459,749 |  |  |  |  |  |
Common Class L | RSU |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  | $ 2,131 |  | $ 2,131 |  |  |  |  |  |
Common Class L | PSU |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  | 467 |  | 467 |  |  |  |  |  |
Common Class L | Restricted Common Stock Units RSU |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  | $ 73 |  | $ 73 |  |  |  |  |  |
Common Class L | Share BasedP ayment Arrangement If Initial Public Offering Occurs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated vesting, percentage |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
Accelerated vesting, minimum percentage of awards unvested |  |  |  | 25.00% |  |  |  |  |  |  |  |  |
Common Class L | Share Based Payment Arrangement If A Change In Control Occurs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Accelerated vesting, percentage |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Common Class L | IPO |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Conversion rate | 13.010 |  |  |  |  |  |  |  |  |  |  |  |
Shares returned for tax payments made on behalf of holders of common stock (in shares) | 1,766 |  |  |  |  |  |  |  |  |  |  |  |

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