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Long-term debt - Long-term debt (Details)
$ in Thousands
 12 Months Ended
Sep. 30, 2025
CAD ($)
tranche
Sep. 30, 2025
USD ($)
Mar. 31, 2025
CAD ($)
Sep. 30, 2024
CAD ($)
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 3,637,835,000     $ 2,688,308,000
Current portion 845,253,000     999,000
Long-term debt 2,792,582,000     2,687,309,000
2021 U.S. Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 1,386,564,000     1,342,758,000
Number of notes | tranche 2      
2021 U.S. Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 1,000,000,000      
Borrowings less financing fees $ 1,393,100,000      
2021 U.S. Senior Notes | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, maturity term 3 years      
Borrowings, interest rate 1.79% 1.79%    
2021 U.S. Senior Notes | Less than one year        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 835,860,000 $ 600,000    
2021 U.S. Senior Notes | Beyond five years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 557,240,000 $ 400,000    
2021 CAD Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 597,892,000     597,212,000
Borrowings, interest rate 2.10% 2.10%    
2021 CAD Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 600,000,000      
2021 CAD Senior Notes | Later than two years and not later than three years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 600,000,000      
2024 CAD Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings 747,001,000     746,144,000
2024 CAD Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 750,000,000      
2024 CAD Senior Notes | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, interest rate 4.08% 4.08%    
2024 CAD Senior Notes | Later than one year and not later than two years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 300,000,000      
2024 CAD Senior Notes | Later than three years and not later than four years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 450,000,000      
2025 U.S. Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings 894,509,000     0
2025 U.S. Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Borrowings less financing fees     $ 650,000,000  
2025 U.S. Senior Notes | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, interest rate     4.95%  
2025 U.S. Senior Notes | Later than four years and not later than five years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 905,515,000 $ 650,000    
Other long-term debt        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 11,869,000     $ 2,194,000