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Capital stock - Narrative (Details) |  |  |  | 12 Months Ended
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Nov. 05, 2024 $ / shares | Feb. 23, 2024 shares | Jan. 30, 2024 shares | Sep. 30, 2024 CAD ($) shares vote | Sep. 30, 2023 CAD ($) shares | Sep. 30, 2022 CAD ($)
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts |  |  |  | $ (15,407,000) | $ (13,879,000) |
Withholding taxes remitted on the net settlement of performance share units |  |  |  | 15,407,000 | 13,879,000 |
Purchase of treasury shares |  |  |  | 66,847,000 | 74,455,000 |
Exercise of stock options |  |  |  | 76,535,000 | 88,316,000 |
Purchase and cancellation of Class A subordinate voting shares |  |  |  | 934,765,000 | $ 788,020,000 |
Net tax liability related to shares repurchased and issued |  |  |  | $ 13,565,000 |  |
Major ordinary share transactions |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Dividends proposed or declared before financial statements authorised for issue but not recognised as distribution to owners per share | $ / shares | $ 0.15 |  |  |  |  |
PSUs |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts (in shares) | shares |  |  |  | 280,265 | 294,203 |
Capital stock |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts (in shares) | shares |  |  |  | 0 | 0 |
Release of shares held in trusts |  |  |  | $ 14,078,000 | $ 13,680,000 |
Purchase of treasury shares |  |  |  | 66,847,000 | 74,455,000 |
Exercise of stock options |  |  |  | $ 91,800,000 | $ 106,051,000 |
Purchased and cancelled (in shares) | shares |  |  |  | 6,597,158,000 | 6,234,096,000 |
Purchased and cancelled |  |  |  | $ 45,878,000 | $ 61,265,000 |
Reclassified between classes of share capital (in shares) | shares |  |  |  | 0 |  |
Capital stock | PSUs |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts (in shares) | shares |  |  |  | 171,751 | 172,018 |
Contributed surplus |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts |  |  |  | $ (30,308,000) | $ (24,674,000) |
Exercise of stock options |  |  |  | (15,265,000) | (17,735,000) |
Retained earnings |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Release of shares held in trusts |  |  |  | $ 823,000 | $ (2,885,000) |
Preferred shares | Capital stock |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Number of votes per share | vote |  |  |  | 1 |  |
Class A subordinate voting shares | Capital stock |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Number of votes per share | vote |  |  |  | 1 |  |
Release of shares held in trusts (in shares) | shares |  |  |  | 0 | 0 |
Release of shares held in trusts |  |  |  | $ 14,078,000 | $ 13,680,000 |
Shares purchased (in shares) | shares |  |  |  | 463,364 | 640,052 |
Purchase of treasury shares |  |  |  | $ 66,847,000 | $ 74,455,000 |
Exercise of stock options |  |  |  | $ 91,800,000 | $ 106,051,000 |
Purchased and cancelled (in shares) | shares |  |  |  | 6,597,158,000 | 6,234,096,000 |
Purchased and cancelled |  |  |  | $ 45,878,000 | $ 61,265,000 |
Reclassified between classes of share capital (in shares) | shares |  |  |  | (2,322,948,000) |  |
Class A subordinate voting shares | Capital stock | Current normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Number of shares authorized for purchase and cancellation (in shares) | shares |  |  | 20,457,737 |  |  |
Purchased and cancelled (in shares) | shares |  | 1,674,930 |  | 1,674,930 |  |
Purchased and cancelled |  |  |  | $ 250,000,000 |  |
Share issue related cost |  |  |  | $ 370,000 |  |
Class A subordinate voting shares | Capital stock | Previous and current normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled (in shares) | shares |  |  |  | 1,965,800 | 2,857,550 |
Purchased and cancelled |  |  |  | $ 275,218,000 | $ 386,906,000 |
Class A subordinate voting shares | Capital stock | Normal course issuer bid - Caisse de depot |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled (in shares) | shares |  |  |  | 2,887,878 | 3,344,996 |
Purchased and cancelled |  |  |  | $ 400,000,000 | $ 400,000,000 |
Class A subordinate voting shares | Capital stock | Previous normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled (in shares) | shares |  |  |  | 68,550 | 100,100 |
Purchased and cancelled |  |  |  | $ 558,000 | $ 778,000 | $ 10,291,000
Purchase and cancellation of Class A subordinate voting shares |  |  |  | $ 9,177,000 |  |
Class A subordinate voting shares | Capital stock | Co-Founder and Advisor to the Executive Chairman of the Board |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Reclassified between classes of share capital (in shares) | shares |  |  |  | (900,000) |  |
Class A subordinate voting shares | Capital stock | Company Wholly-owned by the Founder and Executive Chairman of the Board |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Reclassified between classes of share capital (in shares) | shares |  |  |  | (1,422,948) |  |
Class A subordinate voting shares | Capital stock | PSUs |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Shares outstanding, held in trust (in shares) | shares |  |  |  | 2,601,356 | 2,309,743 |
Class A subordinate voting shares | Contributed surplus |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Exercise of stock options |  |  |  | $ 15,265,000 | $ 17,735,000 |
Class A subordinate voting shares | Retained earnings | Current normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled |  |  |  | 244,821,000 |  |
Class A subordinate voting shares | Retained earnings | Previous and current normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled |  |  |  | 258,883,000 | 363,747,000 |
Class A subordinate voting shares | Retained earnings | Normal course issuer bid - Caisse de depot |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled |  |  |  | $ 375,636,000 | $ 361,791,000 |
Class B shares (multiple voting) | Capital stock |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Number of votes per share | vote |  |  |  | 10 |  |
Release of shares held in trusts (in shares) | shares |  |  |  | 0 | 0 |
Release of shares held in trusts |  |  |  | $ 0 | $ 0 |
Purchase of treasury shares |  |  |  | 0 | 0 |
Exercise of stock options |  |  |  | $ 0 | $ 0 |
Purchased and cancelled (in shares) | shares |  |  |  | 0 | 0 |
Purchased and cancelled |  |  |  | $ 0 | $ 0 |
Reclassified between classes of share capital (in shares) | shares |  |  |  | 2,322,948,000 |  |
Class B shares (multiple voting) | Capital stock | Co-Founder and Advisor to the Executive Chairman of the Board |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Reclassified between classes of share capital (in shares) | shares |  |  |  | 900,000 |  |
Class B shares (multiple voting) | Capital stock | Company Wholly-owned by the Founder and Executive Chairman of the Board |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Reclassified between classes of share capital (in shares) | shares |  |  |  | 1,422,948 |  |
Class B multiple voting shares converted to Class A subordinate voting shares | Capital stock | Current normal course issuer bid |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |
Purchased and cancelled (in shares) | shares |  |  |  | 1,266,366 |  |

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