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Employee benefits - Sensitivity analysis (Details) - Defined benefit obligation
$ in Millions
Dec. 31, 2017
USD ($)
Employee benefits  
Increase of 1% 1.00%
Decrease of 1% 1.00%
Discount rate  
Employee benefits  
Increase in actuarial assumption $ (78.3)
Decrease in actuarial assumption 96.6
Future salary growth  
Employee benefits  
Increase in actuarial assumption 0.2
Decrease in actuarial assumption (0.2)
Health care trends rate  
Employee benefits  
Increase in actuarial assumption 3.2
Decrease in actuarial assumption (2.5)
Future mortality  
Employee benefits  
Increase in actuarial assumption (2.1)
Decrease in actuarial assumption $ 2.1