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Restructuring, acquisition and related integration costs - Narrative (Details) - CAD ($)
 12 Months Ended 18 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Mar. 31, 2024
Disclosure of attribution of expenses by nature to their function [line items]      
Restructuring $ 196,796,000 $ 0  
Cost optimization program 0 91,063,000 $ 100,027,000
Acquisition and related integration costs 88,235,000 5,866,000  
Professional fees 268,322,000 293,452,000  
Restructuring      
Disclosure of attribution of expenses by nature to their function [line items]      
Termination of employment costs 190,008,000    
Cost of vacating leased premises 6,788,000    
Cost optimization program      
Disclosure of attribution of expenses by nature to their function [line items]      
Termination of employment costs   69,500,000  
Cost of vacating leased premises   21,563,000  
Acquisition and related integration costs      
Disclosure of attribution of expenses by nature to their function [line items]      
Termination of employment costs 27,467,000 653,000  
Cost of vacating leased premises 23,379,000 947,000  
Professional fees $ 12,278,000 $ 2,437,000