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Stock-Based Compensation - Narrative (Details) - CAD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Stock-based compensation expense $ 34 $ 14  
Employee Stock Options      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Number of options issued (shares) 215,537    
Weighted average grant price (per share) $ 271.84    
Expiration period 7 years    
Grant date fair value, options $ 14    
Employee Stock Options | Minimum [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 12 months    
Employee Stock Options | Maximum [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 48 months    
Performance Share Units      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Units issued 127,431    
PSU payout percentage     177.00%
Number of trading days     30 days
PSU payout $ 54    
Units vested in the period 117,228    
Performance Share Units | Regular and Retention Grant [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 3 years    
Units issued 128,010    
Grant date fair value, other than options $ 34    
Performance Share Units | Retention Grant [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Units issued 579    
Deferred Share Units [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Vesting period 48 months    
Units issued 13,179    
Grant date fair value, other than options $ 3