Post Employment Plans - Net periodic pension cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	$ 2.7	$ 2.5	$ 3.0
Interest cost	2.8	3.8	3.8
Expected return on assets	(4.9)	(4.9)	(5.3)
Settlement	0.0	0.9	0.0
Curtailment	0.0	(2.5)	(1.2)
Net actuarial loss/(gain)	0.9	0.8	0.6
Net periodic pension cost/(gain)	1.5	0.6	0.9
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service costs	0.0	0.6	0.6
Interest cost	0.1	0.2	0.2
Expected return on assets	0.0	0.0	0.0
Settlement	0.0	0.0	0.0
Curtailment	0.0	0.0	0.0
Net actuarial loss/(gain)	(3.2)	(0.3)	(0.1)
Net periodic pension cost/(gain)	$ (3.1)	$ 0.5	$ 0.7