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Common Stock and Stock-Based Compensation - Narrative (Details) |  |  |  |  | 1 Months Ended | 7 Months Ended | 12 Months Ended |
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Feb. 02, 2021 USD ($) shares | Nov. 02, 2020 USD ($) $ / shares shares | Oct. 02, 2020 USD ($) shares | Sep. 30, 2020 USD ($) | Oct. 31, 2017 USD ($) class $ / shares shares | Nov. 01, 2020 $ / shares shares | Mar. 26, 2021 USD ($) $ / shares shares | Mar. 27, 2020 USD ($) class $ / shares shares | Mar. 29, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Estimated offering costs | $ |  |  |  |  |  |  | $ 1,790,000 |  |
Number of classes of stock | class |  |  |  |  | 2 |  |  | 2 |
Common stock, shares authorized (in shares) |  |  |  |  |  |  | 1,000,000,000 | 0 |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 0.01 | $ 0.01 |
Stock repurchased during period, aggregate purchase price | $ |  |  |  |  |  |  | $ 27,707,000 |  |
Shares issued upon conversion, including shares withheld for tax withholding obligation (in shares) |  | 166,500,000 |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 189,588,161 | 0 |
Shares issued upon conversion (in shares) |  | 164,431,766 |  |  |  |  |  |  |
Weighted average conversion price (in dollars per share) | $ / shares |  | $ 14.00 |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  |  |  | $ 49,870,000 | $ 1,435,000 |
IPO |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | 0 | 28,750,000 |  |  |  |  |  |  |
Offering price (in dollars per share) | $ / shares |  | $ 14.00 |  |  |  |  |  |  |
Net proceeds | $ | $ 0 | $ 321,425,000 |  |  |  |  |  |  |
Underwriting discounts | $ |  | 20,125,000 |  |  |  |  |  |  |
Estimated offering costs | $ |  | $ 8,450,000 |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  |  |  | $ 40,440,000 |  |
IPO | Allegro Microsystems, Inc. |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) |  | 25,000,000 |  |  |  |  |  |  |
IPO - Selling Shareholders |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | 19,332,852 | 3,750,000 |  |  |  |  |  |  |
Shares Of Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 163,972,017 |  |  |  |  |  |  |
Shares Of Unvested Restricted Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 459,749 |  |  |  |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grants in period (in shares) |  |  |  |  |  |  | 1,428,932 |  |
Grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 14.06 |  |
Stock-based compensation expense not yet recorded | $ |  |  |  |  |  |  | $ 13,745,000 |  |
Stock-based compensation expense not yet recorded, period for recognition |  |  |  |  |  |  | 1 year 8 months 12 days |  |
Vested during period (in shares) |  |  |  |  |  |  | 160,063 |  |
Stock-based compensation | $ |  | $ 1,028,000 |  |  |  |  | $ 5,729,000 |  |
Outstanding balance (in shares) |  |  |  |  |  |  | 1,225,156 | 0 | 0
Weighted-average grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 14.07 | $ 0 | $ 0
Performance Stock Units (PSUs) |  |  |  |  |  |  |  |  |
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 |  |
Stock-based compensation expense not yet recorded | $ |  |  |  |  |  |  | $ 8,519,000 |  |
Stock-based compensation expense not yet recorded, period for recognition |  |  |  |  |  |  | 2 years 7 months 24 days |  |
Vested during period (in shares) |  |  |  |  |  |  | 0 |  |
Performance period |  |  |  |  |  |  | 3 years |  |
Intrinsic value, vested | $ |  |  |  |  |  |  | $ 16,433,000 |  |
Stock-based compensation | $ |  |  |  |  |  |  | $ 1,269,000 |  |
Outstanding balance (in shares) |  |  |  |  |  |  | 650,302 | 0 | 0
Weighted-average grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 15.05 | $ 0 | $ 0
Performance Stock Units (PSUs) | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grant percentage of target shares granted maximum |  |  |  |  |  |  | 0 |  |
Inclusion percentage of target goals |  |  |  |  |  |  | 100.00% |  |
Performance Stock Units (PSUs) | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grant percentage of target shares granted maximum |  |  |  |  |  |  | 2 |  |
Inclusion percentage of target goals |  |  |  |  |  |  | 200.00% |  |
Employee Stock Purchase Plan (ESPP) |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock-based compensation expense not yet recorded | $ |  |  |  |  |  |  | $ 247,000 |  |
Stock-based compensation expense not yet recorded, period for recognition |  |  |  |  |  |  | 3 months |  |
Common stock reserved for future issuance (in shares) |  | 1,545,891 |  |  |  |  |  |  |
Number of shares immediately available for grant (in shares) |  | 832,400 |  |  |  |  |  |  |
Number of shares that may become available for grant (in shares) |  | 713,491 |  |  |  |  |  |  |
Purchase price of common stock, percentage of fair market value |  | 85.00% |  |  |  |  |  |  |
Offering period |  | 6 months |  |  |  |  |  |  |
Stock-based compensation | $ |  |  |  |  |  |  | $ 247,000 |  |
Restricted Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grants in period (in shares) |  |  |  |  |  |  | 459,749 |  |
Grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 14.00 |  |
Vested during period (in shares) |  |  |  |  |  |  | 50,170 |  |
Stock-based compensation | $ |  |  |  |  |  |  | $ 174,000 |  |
Outstanding balance (in shares) |  |  |  |  |  |  | 406,327 | 0 | 0
Weighted-average grant date fair value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 14.00 | $ 0 | $ 0
Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchased during period (in shares) |  |  |  |  |  |  | 2,068,274 |  |
Stock repurchased during period, aggregate purchase price | $ |  |  |  |  |  |  | $ 21,000 |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 189,588,161 | 0 |
Common Stock, Class A |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) |  |  |  |  | 2,880,000 |  |  |  |
Net proceeds | $ |  |  |  |  | $ 291,000,000 |  |  |  |
Estimated offering costs | $ |  |  |  |  | $ 9,260,000 |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  | 12,500,000 |  | 0 | 12,500,000 |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  | $ 0.01 |  | $ 0.01 | $ 0.01 |
Issuance of common stock, gross (in shares) |  |  |  |  | 6,720,000 |  |  |  |
Priority dividend, percentage |  |  |  |  | 0.08 |  |  |  |
Payments of dividends | $ |  |  |  | $ 400,000,000 |  |  |  |  |
Grants in period (in shares) |  |  |  |  | 400,000 |  |  |  |
Vesting period |  |  |  |  | 60 months |  |  |  |
Conversion rate |  | 15.822 |  |  |  |  |  |  |
Shares returned for tax payments made on behalf of holders of common stock (in shares) |  | 2,066,468 |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 0 | 10,000,000 |
Shares issued upon conversion (in shares) |  | 156,155,403 |  |  |  |  |  |  |
Stock-based compensation | $ |  | $ 40,440,000 |  |  |  |  |  |  |
Common Stock, Class A | Shares Of Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 156,155,403 |  |  |  |  |  |  |
Common Stock, Class A | Shares Of Unvested Restricted Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 0 |  |  |  |  |  |  |
Common Stock, Class A | OEP |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Noncontrolling interest ownership percentage |  |  |  |  | 28.80% |  |  |  |
Common Stock, 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 | $ |  |  |  |  | $ 40,440,000 |  |  |  |
Accelerated vesting, number (in shares) |  |  |  |  | 400,000 |  |  |  |
Accelerated vesting, share price (in dollars per share) | $ / shares |  |  |  |  | $ 101.10 |  |  |  |
Common Stock, Class A | Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 0 | 10,000,000 | 10,000,000
Common Stock, Class L |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  | 1,000,000 |  | 0 | 1,000,000 |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  | $ 0.01 |  | $ 0.01 | $ 0.01 |
Grants in period (in shares) |  |  |  |  | 597,400 |  |  |  |
Vesting period |  |  |  |  | 4 years |  |  |  |
Accelerated vesting, percentage |  | 0.25 |  |  |  |  |  |  |
Stock repurchased during period (in shares) |  |  | 1,997 |  |  |  |  |  |
Stock repurchased during period, aggregate purchase price | $ |  |  | $ 408,000 |  |  |  |  |  |
Conversion rate |  | 13.010 |  |  |  |  |  |  |
Shares returned for tax payments made on behalf of holders of common stock (in shares) |  | 1,766 |  |  |  |  |  |  |
Outstanding loan amounts extinguished | $ |  |  | $ 753,000 |  |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  | 638,298 | 0 | 622,470 |
Common stock, shares outstanding, weighted average price per share (in dollars per share) | $ / shares |  |  |  |  |  | $ 11.99 |  |  |
Shares issued upon conversion (in shares) |  | 8,276,363 |  |  |  |  |  |  |
Stock-based compensation | $ |  | $ 1,610,000 |  |  |  |  | $ 1,169,000 |  |
Common Stock, Class L | Shares Of Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 7,816,614 |  |  |  |  |  |  |
Common Stock, Class L | Shares Of Unvested Restricted Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued upon conversion (in shares) |  | 459,749 |  |  |  |  |  |  |
Common Stock, Class L | If Initial Public Offering Occurs |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Accelerated vesting, percentage |  |  |  |  | 0.25 |  |  |  |
Accelerated vesting, minimum percentage of awards unvested |  |  |  |  | 0.25 |  |  |  |
Common Stock, Class L | If a Change in Control Occurs |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Accelerated vesting, percentage |  |  |  |  | 1 |  |  |  |
Common Stock, Class L | Common Stock |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares issued during period (in shares) |  |  |  |  |  | 17,203 |  | 30,300 |
Shares issued during period, weighted average price per share (in dollars per share) | $ / shares |  |  |  |  |  | $ 33.83 |  | $ 26.93 |
Stock repurchased during period (in shares) |  |  |  |  |  |  | 1,997 |  |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 0 | 622,470 | 607,620

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