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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of classes of share capital [line items]      
Deficit $ (1,455) $ (1,496)  
NCIB Program      
Disclosure of classes of share capital [line items]      
Number of shares issued and fully paid 7,716,300 3,264,500  
Par value per share $ 8.80 $ 7.02  
Issued capital (Note 24) $ 68 $ 23  
Weighted average share price 83 35  
Deficit 15 12  
Common shares      
Disclosure of classes of share capital [line items]      
Issued capital (Note 24) $ 2,978 $ 3,059 $ 3,094