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Long-term debt - Long-term debt (Details)
 12 Months Ended
Sep. 30, 2023
CAD ($)
tranche
payment
Sep. 30, 2023
USD ($)
tranche
payment
Sep. 30, 2022
CAD ($)
Sep. 30, 2023
USD ($)
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 3,100,321,000   $ 3,267,034,000  
Current portion 1,158,971,000   93,447,000  
Long-term debt 1,941,350,000   3,173,587,000  
Repayments of borrowings 79,150,000   401,654,000  
Settled cross-currency swap agreement        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 69,300,000      
2014 U.S. Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 473,808,000   $ 550,177,000  
Borrowings, number of tranches | tranche 2 2    
Borrowings, number of yearly payments | payment 7 7    
2014 U.S. Senior Notes | Tranche six        
Disclosure of detailed information about borrowings [line items]        
Repayments of borrowings   $ 50,000,000    
2014 U.S. Senior Notes | Less than one year | Remaining total tranches        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 473,830,000     $ 350,000,000
2014 U.S. Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 473,830,000      
2014 U.S. Senior Notes | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, maturity term 1 year 1 year    
Borrowings, interest rate 4.01%   3.98% 4.01%
2014 U.S. Senior Notes | Settled cross-currency swap agreement | Tranche six        
Disclosure of detailed information about borrowings [line items]        
Repayments of borrowings $ 67,765,000      
2021 U.S. Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 1,342,714,000   $ 1,361,974,000  
Borrowings, number of tranches | tranche 2 2    
2021 U.S. Senior Notes | Later than one year and not later than five years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 812,280,000     $ 600,000,000
2021 U.S. Senior Notes | Beyond five years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities 541,520,000     $ 400,000,000
2021 U.S. Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 1,353,800,000      
2021 U.S. Senior Notes | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, maturity term 5 years 5 years    
Borrowings, interest rate 1.79%     1.79%
2021 CAD Senior Notes        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 596,550,000   595,900,000  
Borrowings, interest rate 2.10%     2.10%
2021 CAD Senior Notes | Later than one year and not later than five years        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities       $ 600,000,000
2021 CAD Senior Notes | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 600,000,000      
Unsecured committed term loan credit facility        
Disclosure of detailed information about borrowings [line items]        
Borrowings 676,886,000   687,705,000  
Unsecured committed term loan credit facility | Gross carrying amount        
Disclosure of detailed information about borrowings [line items]        
Notional amount of financial liabilities $ 676,900,000      
Unsecured committed term loan credit facility | LIBOR | Weighted average        
Disclosure of detailed information about borrowings [line items]        
Borrowings, interest rate 6.43%     6.43%
Other long-term debt        
Disclosure of detailed information about borrowings [line items]        
Borrowings $ 10,363,000   $ 71,278,000