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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of classes of share capital [line items]      
Deficit $ (1,496) $ (1,209)  
NCIB Program      
Disclosure of classes of share capital [line items]      
Number of shares issued and fully paid 3,264,500    
Par value per share $ 7.02    
Issued capital (Note 24) $ 23    
Weighted average share price 35    
Deficit 12    
Common shares      
Disclosure of classes of share capital [line items]      
Issued capital (Note 24) $ 3,059 $ 3,094 $ 3,095