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Financial instruments - Narrative (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2025 CAD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 CAD ($)
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Borrowings | $ 3,637,835,000 |  | $ 2,688,308,000
Cash and cash equivalents, funds held for clients, short-term investments and long-term investments | 1,874,007,000 |  | 1,995,413,000
Available credit facility | 1,496,248,000 |  | 1,496,355,000
Trade receivables | 1,343,282,000 |  | 1,117,712,000
Cross-currency swap agreement between USD to CAD |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount of financial liabilities |  | $ 650,000,000 |
Cross-currency swap agreement between USD to Swedish krona |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount of financial liabilities |  | 63,500,000 |
Cross-currency swap agreement between USD to Danish krone |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount of financial liabilities |  | $ 11,500,000 |
Unsecured committed term loan credit facility |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Borrowings | 0 |  |
2021 U.S. Senior Notes |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Borrowings | $ 1,386,564,000 |  | $ 1,342,758,000
Interest rate risk |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Sensitivity analysis for types of market risk, reasonably possible change in risk variable, percent | 1.00% |  |
Currency risk |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Sensitivity analysis for types of market risk, reasonably possible change in risk variable, percent | 10.00% |  |
Currency risk | Hedges of net investments in European operations |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount of financial liabilities | $ 1,393,100,000 |  |
Currency risk | Cash flow hedges on the 2025 U.S. Senior Notes | Cross-currency swap agreement between USD to CAD |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Receive Rate | 495.00% | 495.00% |
Currency risk | Cash flow hedges on the 2025 U.S. Senior Notes | Cross-currency swap agreement between USD to CAD | Maximum |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Receive Rate | 4.95% | 4.95% |
Currency risk | Cash flow hedges on the 2025 U.S. Senior Notes | Cross-currency swap agreement between USD to CAD | Minimum |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Receive Rate | 3.71% | 3.71% |
Currency risk | 2021 U.S. Senior Notes |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Borrowings | $ 1,386,564,000 |  |

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