Defined benefit and 401(k) savings plan - Components of Net Periodic Pension Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Retirement Benefits [Abstract]
Service cost	$ 2.4	$ 1.9	$ 1.3
Interest cost	0.0	0.1	0.2
Expected return on plan assets	(0.8)	(0.6)	(0.5)
Amortization of loss	0.6	0.2	0.0
Amortization of prior service cost	(0.2)	(0.2)	0.0
Settlements	0.0	1.0	0.5
Net periodic benefit cost	$ 2.0	$ 2.4	$ 1.5