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Post-employee benefit plans - Components of Post-Employment Benefit Plans Financing Cost (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [line items]    
Total interest on post-employment benefit obligations $ (46) $ (63)
DB pension    
Disclosure of net defined benefit liability (asset) [line items]    
Total interest on post-employment benefit obligations (10) (19)
OPEBs    
Disclosure of net defined benefit liability (asset) [line items]    
Total interest on post-employment benefit obligations $ (36) $ (44)