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Share Capital - Common Shares Issued and Outstanding (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
shares
$ / shares
Dec. 31, 2017
CAD ($)
shares
Aug. 14, 2018
shares
Aug. 14, 2017
shares
Reconciliation of number of shares outstanding [abstract]        
Stock options exercised (in shares) 399,000 437,000    
Common shares issued as consideration for business acquisition (in shares) 100,000 0    
Common shares issued as consideration for business acquisition | $ $ 4 $ 0    
Payments to acquire or redeem entity's shares | $ $ 641 $ 175    
Common shares        
Reconciliation of number of shares outstanding [abstract]        
Number of shares outstanding, beginning of year (in shares) 610,500,000 613,600,000    
Stock options exercised (in shares) 400,000 400,000    
Common shares purchased for cancellation (in shares) 12,500,000.0 (3,500,000)    
Number of shares outstanding, end of year (in shares) 598,500,000 610,500,000    
Balance, beginning of year | $ $ 8,582 $ 8,614    
Stock options exercised | $ 16 18    
Common shares purchased for cancellation | $ (183) (50)    
Balance, end of year | $ $ 8,419 $ 8,582    
Number of shares authorized for purchase and cancellation (in shares)     14,000,000 11,500,000.0
Purchase and cancellation of treasury shares (cad per share) | $ / shares $ 50.83      
Payments to acquire or redeem entity's shares | $ $ 635      
2018 NCIB | Common shares        
Reconciliation of number of shares outstanding [abstract]        
Common shares purchased for cancellation (in shares) 100,000.0      
Purchase and cancellation of treasury shares (cad per share) | $ / shares $ 44.91      
Payments to acquire or redeem entity's shares | $ $ 0      
Third-Party Share Repurchase Program | Common shares        
Reconciliation of number of shares outstanding [abstract]        
Common shares purchased for cancellation (in shares)   1,100,000.0