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Employee benefits - Sensitivity Analysis (Details) - Defined benefit plan
 12 Months Ended
Dec. 31, 2018
Discount rate  
Sensitivity of the balance of actuarial liabilities  
Change (as a percent) 0.50%
Increase in liability (as a percent) (5.67%)
Decrease in liability (as a percent) 6.25%
Medical cost growth rate  
Sensitivity of the balance of actuarial liabilities  
Change (as a percent) 0.50%
Increase in liability (as a percent) 6.41%
Decrease in liability (as a percent) (5.86%)
Mortality  
Sensitivity of the balance of actuarial liabilities  
Change (in years) 1 year
Increase in liability (as a percent) 0.47%
Decrease in liability (as a percent) (2.27%)