Defined benefit plan (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Retirement Benefits [Abstract]
Net service cost	$ 0.3	$ 0.6
Expected return on plan assets, net of expenses	(0.1)	(0.1)
Total pension expense	$ 0.2	$ 0.5