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Significant accounting policies - Adoption of New or Amended Accounting Standards (Details) - CAD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Other current assets   $ (324) $ (253)  
Cash   99 289 $ 224
Capital expenditures   (5,133) (4,852)  
Other financing activities   (31) 19  
Government Of Québec        
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Increase in deficit $ (97)      
Cumulative Effect, Period of Adoption, Adjustment        
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Other current assets     $ 82  
Cash   82    
Capital expenditures   (15)    
Other financing activities   $ 97