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Common Shares - The effects of the Corporation's purchase and cancellation of the common shares (Details) - CAD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
May 25, 2021
May 26, 2020
Dec. 31, 2019
Disclosure of classes of share capital [line items]          
Deficit $ (2,453) $ (1,826)      
Percentage of public float of common shares making up stock purchase agreement, shares authorized     7.16% 7.02%  
Number of shares issued and fully paid that were not cancelled due to timing differences between the transaction date and settlement date 0 456,200      
NCIB Program          
Disclosure of classes of share capital [line items]          
Number of shares issued and fully paid 0 7,352,600      
Par value per share $ 0 $ 8.33      
Issued capital $ 0 $ 61      
Weighted average book value of shares cancelled 0 79      
Deficit 0 18      
Stock purchase agreement, number of shares authorized     14,000,000 14,000,000  
Common shares          
Disclosure of classes of share capital [line items]          
Issued capital $ 2,901 $ 2,896     $ 2,978