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Pension and post-retirement benefits - Summary of Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Pensions    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost $ 0.3 $ 0.6
Interest cost 0.2 0.4
Expected return on plan assets (0.2) (0.4)
Net periodic benefit cost (income) 0.3 0.6
Other Benefits    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]    
Service cost 0.0 0.0
Interest cost 0.0 0.0
Expected return on plan assets 0.0 0.0
Net periodic benefit cost (income) $ 0.0 $ 0.0