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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 |  |

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