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Common Stock and Stock-Based Compensation - Narrative (Details)
 1 Months Ended 7 Months Ended 12 Months Ended
Feb. 02, 2021
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Nov. 02, 2020
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$ / shares
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Oct. 02, 2020
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shares
Sep. 30, 2020
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Oct. 31, 2017
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$ / shares
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Nov. 01, 2020
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Mar. 26, 2021
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Mar. 27, 2020
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Mar. 29, 2019
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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