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Employee Future Benefits - Defined Benefit Obligation (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period $ 278    
Current service cost 8 $ 10 $ 12
Actuarial gain arising from demographic assumptions (11) (26)  
Defined benefit obligation, ending of period 294 278  
Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 664    
Interest cost 20 23 22
Defined benefit obligation, ending of period 675 664  
Registered      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 170    
Current service cost 5 7 9
Defined benefit obligation, ending of period 175 170  
Registered | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 543 514  
Current service cost 5 7  
Interest cost 16 19 18
Benefits paid (45) (51)  
Curtailment and amendment gain (2) (3)  
Actuarial gain arising from demographic assumptions 0 0  
Actuarial loss arising from financial assumptions 43 57  
Actuarial gain (loss) arising from experience adjustments (17) 2  
Effect of translation on US plans (1) (2)  
Defined benefit obligation, ending of period 542 543 514
Supplemental Pension Plan      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 86    
Current service cost 2 2 2
Defined benefit obligation, ending of period 95 86  
Supplemental Pension Plan | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 99 80  
Current service cost 2 2  
Interest cost 3 3 3
Benefits paid (5) (4)  
Curtailment and amendment gain 0 0  
Actuarial gain arising from demographic assumptions 0 0  
Actuarial loss arising from financial assumptions 10 9  
Actuarial gain (loss) arising from experience adjustments 0 9  
Effect of translation on US plans 0 0  
Defined benefit obligation, ending of period 109 99 80
Other      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 22    
Current service cost 1 1 1
Defined benefit obligation, ending of period 24 22  
Other | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 22 25  
Current service cost 1 1  
Interest cost 1 1 1
Benefits paid (1) (1)  
Curtailment and amendment gain 0 0  
Actuarial gain arising from demographic assumptions 0 (2)  
Actuarial loss arising from financial assumptions 2 2  
Actuarial gain (loss) arising from experience adjustments 0 (4)  
Effect of translation on US plans (1) 0  
Defined benefit obligation, ending of period 24 22 25
Total | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligation, beginning of period 664 619  
Current service cost 8 10  
Interest cost 20 23  
Benefits paid (51) (56)  
Curtailment and amendment gain (2) (3)  
Actuarial gain arising from demographic assumptions 0 (2)  
Actuarial loss arising from financial assumptions 55 68  
Actuarial gain (loss) arising from experience adjustments (17) 7  
Effect of translation on US plans (2) (2)  
Defined benefit obligation, ending of period $ 675 $ 664 $ 619