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Stock option plan - Additional information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Sep. 01, 2020
Sep. 26, 2018
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of PSUs that may vest based on performance     25.00%    
Unrecognized compensation cost, stock options     $ 26,300    
Unrecognized stock-based compensation expense, period for recognition     1 year 7 months 6 days    
Increase in stock-based compensation expense     $ 36,998 $ 17,096 $ 11,170
Weighted average grant date fair value of stock options granted     $ 42.80 $ 20.76 $ 15.06
Shares subject to right of repurchase     2,741    
2018 ESPP [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Minimum annual increase in share reserved for issuance   249,470      
Annual automatic Increase in share reserved for issuance (as a percent)   1.00%      
Aggregate number of common stock shares reserved for issuance   275,225 355,000 179,000  
Percentage of payroll deduction to purchase common stock   15.00%      
ESPP eligible employee common stock purchase price ratio   85.00%      
2018 Equity Incentive Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Minimum annual increase in share reserved for issuance   999,900      
Annual automatic Increase in share reserved for issuance (as a percent)   4.00%      
Weighted average grant date fair value of stock options granted     $ 69.03    
2018 Equity Incentive Plan [Member] | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Common stock shares may be issued   6,856,405      
Unvested restricted stock units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share based compensation by share based payment arrangement equity instruments other than options non vested outstanding grant date fair value     $ 73.86 $ 32.96  
Unvested restricted stock units | 2018 ESPP [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized compensation cost, stock options     $ 47,500    
Unrecognized stock-based compensation expense, period for recognition     2 years 8 months 12 days    
Phantom Share Units (PSUs) [Member] | 2018 Equity Incentive Plan [Member] | Maximum | Executive Officers and Senior Level Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of PSUs that may vest based on performance 150.00%        
Phantom Share Units (PSUs) [Member] | 2018 Equity Incentive Plan [Member] | Minimum | Executive Officers and Senior Level Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of PSUs that may vest based on performance 0.00%        
Phantom Share Units (PSUs) [Member] | Revised Annual Equity Award Program [Member] | Executive Officers and Senior Level Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share based compensation by share based payment arrangement instruments other than options expected to vest outstanding     256,665    
Share based compensation by share based payment arrangement equity instruments other than options non vested outstanding grant date fair value     $ 45.18    
Share based compensation by share based payment arrangement equity instruments other than options unrecognized compensation     $ 5,600    
Unrecognized stock-based compensation expense, period for recognition     1 year    
Phantom Share Units (PSUs) [Member] | Revised Annual Equity Award Program [Member] | Executive Officers and Senior Level Employees [Member] | PSU Grants [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Increase in stock-based compensation expense     $ 600