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Financial instruments - Sensitivity analysis (Details) - Currency risk - CAD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
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% |
euro impact |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Increase in net earnings | $ 2,806 | $ 150
Decrease in other comprehensive income (loss) | (211,271) | (174,239)
U.S. dollar impact |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Increase in net earnings | 5,827 | 1,359
Decrease in other comprehensive income (loss) | (235,753) | (180,405)
British pound impact |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Increase in net earnings | 990 | 1,179
Decrease in other comprehensive income (loss) | (22,262) | (17,269)
Swedish krona impact |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Increase in net earnings | 265 | 521
Decrease in other comprehensive income (loss) | $ (20,296) | $ (9,631)

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