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Post-employee benefit plans - Contributions (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]    
Contributions, defined contribution plan $ (108) $ (99)
Estimated of contributions expected to be paid to plan for next annual reporting period, defined contribution plan 110  
DB pension    
Disclosure of defined benefit plans [line items]    
Estimate of contributions expected to be paid to plan for next annual reporting period, defined benefit plan 210  
OPEBs    
Disclosure of defined benefit plans [line items]    
Estimate of contributions expected to be paid to plan for next annual reporting period, defined benefit plan 80  
Plan assets    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan (382) (702)
Plan assets | DB pension    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan (305) (626)
Voluntary contributions 100 400
Plan assets | OPEBs    
Disclosure of defined benefit plans [line items]    
Contributions, defined benefit plan $ (77) $ (76)