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Employee Future Benefits - Status of Plans (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ (294) $ (278)  
Accrued current liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (12) (10)  
Other long-term liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (282) (268)  
Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 381 386 $ 381
Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (675) (664)  
Registered      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (175) (170)  
Registered | Accrued current liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (5) (3)  
Registered | Other long-term liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (170) (167)  
Registered | Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 367 373 368
Registered | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (542) (543) (514)
Supplemental Pension Plan      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (95) (86)  
Supplemental Pension Plan | Accrued current liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (5) (5)  
Supplemental Pension Plan | Other long-term liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (90) (81)  
Supplemental Pension Plan | Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 14 13 13
Supplemental Pension Plan | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (109) (99) (80)
Other      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (24) (22)  
Other | Accrued current liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (2) (2)  
Other | Other long-term liabilities      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) (22) (20)  
Other | Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 0 0 0
Other | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ (24) $ (22) $ (25)