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Employee benefits - Sensitivity analysis for actuarial assumptions (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Actuarial assumption of discount rates | U.K.    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ (15,218) $ (18,334)
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ 15,941 $ 19,263
Actuarial assumption of discount rates | France    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ (2,819) $ (2,927)
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ 2,937 $ 3,056
Actuarial assumption of discount rates | GERMANY    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ (1,725) $ (1,796)
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ 1,795 $ 1,874
Actuarial assumption of expected rates of salary increases | U.K.    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 88 $ 181
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (88) $ (179)
Actuarial assumption of expected rates of salary increases | France    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 3,040 $ 3,151
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (2,929) $ (3,029)
Actuarial assumption of expected rates of salary increases | GERMANY    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 20 $ 23
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (18) $ (21)
Actuarial assumption of expected rates of pension increases | U.K.    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 9,573 $ 10,675
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (8,329) $ (9,287)
Actuarial assumption of expected rates of pension increases | France    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 0 $ 0
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ 0 $ 0
Actuarial assumption of expected rates of pension increases | GERMANY    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 924 $ 948
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (893) $ (913)
Actuarial assumption of expected rates of inflation | U.K.    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 10,803 $ 12,047
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (10,577) $ (11,798)
Actuarial assumption of expected rates of inflation | France    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 3,040 $ 3,151
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (2,929) $ (3,029)
Actuarial assumption of expected rates of inflation | GERMANY    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible increase in actuarial assumption 0.25% 0.25%
Percentage of reasonably possible decrease in actuarial assumption 0.25% 0.25%
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 924 $ 948
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (893) $ (913)
Actuarial assumption of life expectancy after retirement | U.K.    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Period of reasonably possible increase in actuarial assumption 1 year 1 year
Period of reasonably possible decrease in actuarial assumption 1 year 1 year
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 13,356 $ 15,309
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (13,750) $ (15,478)
Actuarial assumption of life expectancy after retirement | France    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Period of reasonably possible increase in actuarial assumption 1 year 1 year
Period of reasonably possible decrease in actuarial assumption 1 year 1 year
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 686 $ 664
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (733) $ (710)
Actuarial assumption of life expectancy after retirement | GERMANY    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Period of reasonably possible increase in actuarial assumption 1 year 1 year
Period of reasonably possible decrease in actuarial assumption 1 year 1 year
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption $ 2,044 $ 2,025
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ (1,828) $ (1,809)