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Income taxes - Narrative (Details) - CAD ($)
Sep. 30, 2022
Sep. 30, 2021
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Net deferred tax asset $ 46,893,000 $ 38,371,000
Cash and cash equivalents 966,458,000 1,699,206,000
Operating tax losses carried forward    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Tax losses carried forward 258,244,000 225,002,000
Deferred tax asset is recognised from loss 179,329,000 162,693,000
Non-operating tax losses carried forward    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Tax losses carried forward 421,218,000 469,097,000
Net deferred tax asset 5,070,000 4,810,000
Deferred tax asset is recognised from loss 20,295,000 20,534,000
Unrecognized losses 400,923,000 448,563,000
Expiring at various dates up to 2042 | Operating tax losses carried forward    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Tax losses carried forward 110,918,000 82,548,000
Unrecognized losses 12,450,000 25,325,000
No expiry dates | Operating tax losses carried forward    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Tax losses carried forward 147,326,000 142,454,000
Unrecognized losses 66,466,000 36,984,000
Foreign countries | Subsidiaries    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Cash and cash equivalents 907,577,702 1,420,634,000
Undistributed earnings from foreign subsidiaries for which deferred tax liabilities have not been recorded $ 7,100,148,000 $ 6,290,351,000