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Share Capital - Common Shares Issued and Outstanding (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
shares
$ / shares
Dec. 31, 2019
CAD ($)
shares
Aug. 14, 2019
shares
Reconciliation of number of shares outstanding [abstract]      
Stock options exercised (in shares) 631,000 772,000  
Payments to acquire or redeem entity's shares | $ $ 200 $ 592  
Common shares      
Reconciliation of number of shares outstanding [abstract]      
Number of shares outstanding, beginning of year (in shares) 587,800,000 598,500,000  
Stock options exercised (in shares) 800,000 800,000  
Common shares purchased for cancellation (in shares) (3,500,000) (11,500,000)  
Number of shares outstanding, end of year (in shares) 585,100,000 587,800,000  
Balance, beginning of year | $ $ 8,289 $ 8,419  
Stock options exercised | $ 23 28  
Common shares purchased for cancellation | $ (50) (158)  
Balance, end of year | $ $ 8,262 $ 8,289  
Number of shares authorized for purchase and cancellation (in shares)     15,000,000
Third-Party Share Repurchase Program | Common shares      
Reconciliation of number of shares outstanding [abstract]      
Common shares purchased for cancellation (in shares)   1,100,000  
2019 NCIB | Common shares      
Reconciliation of number of shares outstanding [abstract]      
Common shares purchased for cancellation (in shares) (3,500,000)    
Purchase and cancellation of treasury shares (cad per share) | $ / shares $ 56.86    
Payments to acquire or redeem entity's shares | $ $ 200