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Employee future benefits - Actuarial Assumptions (Details)
Dec. 31, 2018
Dec. 31, 2017
Pension Plan    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Discount rate, fair value benefit obligation 4.16% 3.60%
Discount rate, net expense 3.60% 4.13%
Other post-retirement benefit plan    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Discount rate, fair value benefit obligation 3.96% 3.27%
Discount rate, net expense 3.96% 3.27%
Initial medical/dental health care cost trend rate   8.00%
Other post-retirement benefit plan | Actuarial assumption of medical cost trend rates    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Cost trend rate declines to medical and dental   5.00%