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Share-based payments - Performance share units (Details) - PSUs
 12 Months Ended
Sep. 30, 2023
plan
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]  
Number of plans in share-based payment arrangement | plan 2
Vesting period for PSUs 4 years
Class A subordinate voting shares  
Disclosure of terms and conditions of share-based payment arrangement [line items]  
Number of shares per PSU to which participants are entitled (in shares) | shares 1