Employee benefit plans - Changes in obligations and assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in benefit obligation
Benefit obligation at beginning of the year	$ (14.1)	$ (13.2)
Service cost	(0.3)	(0.3)
Interest cost	(0.4)	(0.5)
Actuarial loss	(2.3)	(2.0)
Employee contributions	(0.1)	(0.1)
Benefits paid	0.3	0.2
Settlements		2.4
Foreign currency adjustment	(0.5)	(0.6)
Benefit obligation at end of year	(17.4)	(14.1)
Change in plan assets
Fair value of plan assets at beginning of the year	12.9	12.0
Actual return on plan assets	1.2	2.0
Employer contributions	0.2	0.8
Employee contributions	0.1	0.1
Benefits paid	(0.3)	(0.2)
Settlements		(2.4)
Foreign currency adjustment	0.2	0.6
Fair value of plan assets at end of year	14.3	12.9
Funded status at end of year - excess of obligation over assets	$ (3.1)	$ (1.2)