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Employee Future Benefits - Costs Recognized (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]      
Current service cost $ 8 $ 10 $ 12
Administration expenses 1 2 1
Defined benefit expense 15 19 22
Defined contribution expense 9 9 10
Net expense 24 28 32
Defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Interest cost 20 23 22
Plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Interest cost (12) (13) (13)
Curtailment and amendment gain (2) (3)  
Registered      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 5 7 9
Administration expenses 1 2 1
Defined benefit expense 9 13 15
Defined contribution expense 9 9 10
Net expense 18 22 25
Registered | Defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 5 7  
Interest cost 16 19 18
Curtailment and amendment gain (2) (3)  
Registered | Plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Interest cost (11) (12) (13)
Curtailment and amendment gain (2) (3)  
Supplemental Pension Plan      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 2 2 2
Administration expenses 0 0 0
Defined benefit expense 4 4 5
Defined contribution expense 0 0 0
Net expense 4 4 5
Supplemental Pension Plan | Defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 2 2  
Interest cost 3 3 3
Curtailment and amendment gain 0 0  
Supplemental Pension Plan | Plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Interest cost (1) (1) 0
Curtailment and amendment gain 0 0  
Other      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 1 1 1
Administration expenses 0 0 0
Defined benefit expense 2 2 2
Defined contribution expense 0 0 0
Net expense 2 2 2
Other | Defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 1 1  
Interest cost 1 1 1
Curtailment and amendment gain 0 0  
Other | Plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Interest cost 0 0 $ 0
Curtailment and amendment gain $ 0 $ 0