Employee Benefits - Schedule of changes in net liability for defined benefit plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in pension benefit obligations:
Balance at beginning of year	$ (182.9)	$ (222.6)
Service cost	(0.3)	0.0
Interest cost	(5.2)	(5.1)	$ (7.2)
Actuarial gains (losses)	(20.8)	17.2
Benefits paid	8.7	14.3
Foreign exchange movement	(8.7)	13.3
Balance at end of year	(209.2)	(182.9)	(222.6)
Change in pension plan assets:
Balance at beginning of year	188.2	213.6
Actual return on plan assets	30.2	(7.5)
Employer contributions	5.3	9.5
Benefits paid	(8.7)	(14.3)
Foreign exchange movement	8.9	(13.1)
Balance at end of year	223.9	188.2	$ 213.6
Net asset/(liability)	$ 14.7	$ 5.3